Contrary to popular belief, a narrator explains, Mars contains life, air, and water. Its inhabitants call it Barsoom. The country of Zodanga, led by the ruthless Sab Than (Dominic West), has overthrown almost every other country on the planet. Only Helium, another country, prevents in Sab Than from total domination of Barsoom. One day, Sab Than’s flying ship is caught in a sandstorm. When all seems lost to him, three figures in gray robes appear. Their leader, Matai Shang, offers Sab Than a glowing webbed blue weapon that slips over his hand and is absorbed into his body. The goddess chosen Sab Than to bear this weapon, Matai Shang explains, and with it Sab Than will be unstoppable.
New York, 1881. John Carter strides down a rainy street. A man in a bowler hat follows him and watches as Carter sends a telegram to his nephew Ned. Later, Ned reads Carter’s telegram on a train. It bids him to come visit his uncle immediately. Unfortunately, when Ned arrives at the train station, his uncle’s butler informs him his uncle died that morning. Ned goes to Carter’s lavish manor, packed with artifacts from the latter’s numerous archaeological digs. Carter’s lawyer comments that Carter always seemed to be looking for something. Ned pays his respects to his uncle’s mausoleum, which can only be opened from the inside. John Carter did not want an open casket funeral and is already inside the mausoleum, which bears the phrase ‘Inter Mundos.’ Carter’s lawyer informs Ned that he has inherited the entirety of his uncle’s estate as well as his uncle’s journal. Ned sits down to read his uncle’s journal, which begins with a missive for Ned to indulge him one last fanciful story.
Carter’s tale begins in Arizona thirteen years before, in 1868. Carter, a prospector, tries to buy supplies at a local tavern. The owner refuses to sell to Carter, citing Carter’s sizeable tab. Apparently Carter has been searching for the Evil Spider’s Cave, which legend holds is full of gold, which the tavern owner and his other patrons find laughable. Carter produces a piece of gold with a nine-legged spider on it; the gold piece will cover Carter’s table and then some, so the tavern owner better help him. When the tavern owner still refuses, he and his patrons get into a fight with Carter. Carter wins the fight, but some cavalrymen, led by General Powell (Bryan Cranston), show up and drag Carter to their fort. His numerous escape attempts land him in a jail cell. Powell, impressed by Captain Carter’s impressive Civil War record, demands that Carter help him fight the Apaches. Carter refuses. While in the cell, Carter flashes back to a memory of his wife; he wears two wedding rings. Eventually he manages to break out of the cell and makes off with Powell’s own horse. Powell and other cavalrymen chase Carter straight into a band of Apache warriors. Carter tries to negotiate with them, but a nervous cavalryman shoots and kills an Apache. The two opposing sides clash, and before Carter can make a clean getaway, a warrior shoots Powell. Carter helps Powell escape, and eventually they are chased to the mouth of a cave. Carter sees the same nine-legged spider symbol at the mouth of the cave and knows he has found his gold. He enters the cave and is shocked when a grey robed man materializes out of a swirling blue light. He shoots the grey robed man, who clutches an ornate silver medallion as he dies. Carter takes the medallion and listens to the dying man mumble a gibberish phrase. “Barsoom?” Carter asks. The medallion glows with a bright blue light, and Carter disappears.
Carter awakens in a desolate desert not dissimilar to the one in Arizona. Except that in this desert, he cannot walk properly: instead, each step he takes sends him flying into the air. Carter drops the medallion but finally figures out how to jump gracefully instead of walking. He reaches a rock formation and hears a rattling sound coming from within. He discovers a clear pane of glass that overlooks a giant nest. Carter recoils when four-armed green aliens begin to hatch. A group of those aliens discovers Carter’s medallion. A fight ensues when the aliens, called Tharks, discovers Carter at their nest. Their leader all leaders on Barsoom are called Jeddaks Tars Tarkas (voiced by Willem Dafoe) is amazed when he sees Carter leap into the air. He introduces himself to Carter, but the two cannot understand each other because they speak different languages. The language barrier leads Tars Tarkas to believe Carter’s name is Virginia, when that’s only from where Carter hails. Tars Tarkas conveys to Carter that he wants him to jump again, and eventually Carter complies he jumps over Tars Tarkas and lands next to the Jeddak’s weapons. The warriors with Tars Tarkas shoot Carter. Tars Tarkas straps Carter to one of their alien horse creatures, along with the Thark hatchlings. He orders his warriors to kill the weak ones.
Dejah Thoris (Lynn Collins), princess of Helium, practices a very important speech. She needs to convince her father, Jeddak Tardos Mors (Ciarán Hinds), that she has harnessed the power of the ninth ray. The ninth ray it is the same force that powers Sab Than’s new weapon has the power to replenish Barsoom’s oceans as well as defeat Sab Than’s approaching army. Her father and his generals, including the valiant Kantos Kan (James Purefoy), arrive, and Dejah turns on a machine that projects the ninth ray. One of the general’s touches the machine he wears the same medallion as the grey robed man and it malfunctions. Tardos Mors explains to his daughter that Sab Than will spare Helium is Dejah marries him. The prospect horrifies Dejah, but her love for her people convinces her to agree. In the hallway, the general who caused the machine to malfunction transforms into a grey robed man. He communicates via a high-tech earpiece with Matai Shang, telling him that he broke the machine. Now Dejah will not be able to harness the ninth ray’s powers.
The Thark warriors and Carter reach the Tharks’ city. More Tharks emerge from the rocks, and the women begin to fight over the hatchlings. Sola (voiced by Samantha Morton) struggles to grab a hatchling, but Thark women like Sarkoja (voiced by Polly Walker) continually beat her. Tars Tarkas commands Sarkoja to allow Sola to have a hatchling, and Sola is made responsible for Carter’s upkeep. Carter obstinately refuses to jump for the Tharks, so Tars Tarkas sends him to be initiated with the other hatchlings. The Thark women bathe and shave Carter before chaining him to a wall. Sola force-feeds Carter a potion, “the voice of Barsoom,” which allows him to understand their language. When he awakens in the night, he breaks free from his chains and attempts to escape. Woola, a sweet and surprisingly adorable dog-lizard creature, finds and attaches himself to Carter. Because Woola can move at warp speed, he is able to keep up with Carter, even when the human makes giant leaps. Carter goes to inspect a Thark party, and Woola follows. When Woola disrupts the party, some Thark warriors begin to beat him. Carter emerges from his hiding place and punches one of the warriors, killing him with once punch. Tars Tarkas, displeased by Carter’s outburst, punishes the human and Sola for allowing Carter to escape. They are chained to a pole, and Sola is branded; Tars Tarkas tells the pair that the next time Sola makes a mistake, she will be killed. The arrival of a flyer one of the ships flown by the humans on Barsoom distracts the Tharks from Carter’s punishment.
Sab Than attacks a ship manned by Helium soldiers. Dejah, who fled her city rather than marry him, is somewhere onboard. As Tars Tarkas and Carter watch the mid-air battle, the former explains that those from Zodanga wear red and are called Red Men. People from Helium wear blue and are called Blue Men. Sab Than’s forces overwhelm the Helium ship, but he cannot find his betrothed amongst the captive fighters. Dejah slips from a hiding spot and tries to escape, but she slips and is left clinging to the edge of her ship. She loses her grip, and Carter jumps through air to catch her. When they land, he tells her to get behind him so he can battle the advancing Red Men. Dejah grabs a sword and demonstrates her prowess in battle. Sab Than and his forces are temporarily defeated and leave. Dejah surrenders to the Tharks, and Tars Tarkas gives her to Carter as a spoil of war. She will be safe as long as Carter agrees to fight for the Tharks and help them dominate their enemies. To protect Dejah, Carter agrees to be their fighter and is made a Thark warrior, much to the displeasure of the mutinous Tal Hajus (voiced by Thomas Haden Church).
Carter learns he’s on Mars by talking with Dejah. She begs him to help her save Helium, but he professes that he wants no part of war. Dejah does not believe Carter comes from Earth, called Jasoom by Dejah, but when he shows her the medallion that brought him to Barsoom, she realizes he might be a Thern. Much to Sola’s distress, Dejah takes Carter into the Thark temple, which is forbidden to non-Tharks. Dejah explains that Therns are the messengers of the Goddess, that they came from Jasoom, and that they helped people on Barsoom “back in the time of the oceans.” If Carter and Dejah travel down the sacred river Is, to the Gates of Is, he might find the answers necessary to return home. She agrees to take Carter to the Gates of Is. Sarkoja discovers Carter, Dejah, and Sola inside the temple. She reports their break in to Tars Tarkas, who is furious at Carter’s betrayal, mostly because he must now sentence Sola to death. Carter discerns, from Tars Tarkas’s concern for Sola’s fate, that she is his daughter. Tars Tarkas decides to allow Carter and Dejah to escape, as long as they take Sola with them. He would rather she die in a sacred place than a Thark fighting pit. Tal Hajus is furious when he discovers that Tars Tarkas helped the trio escape.
Carter, Dejah, and Sola ride lizard-horse creatures through the barren Barsoom landscape. Woola accompanies him, and Sola tells Carter that Woola will follow him anywhere. They pass cities that Sab Than and his army have destroyed. Dejah reiterates that Helium is Barsoom’s last hope. Eventually Sola and Carter deduce that Dejah has been leading them towards Helium, not the River Is. Carter forces Dejah off her lizard-horse and pretends he will abandon her. A desperate Dejah finally admits that she “escaped” Helium because she did not want to marry Sab Than. Carter accuses her of abandoning her people, but he takes pity on her and allows her to rejoin the group. Dejah agrees to take them to the River Is.
Sab Than prepares to leave his ship to travel to Helium. A general advises him against going alone, without a bodyguard, but Sab Than disregards his concern. He believes that meeting Tardos Mors without a bodyguard will be a gesture of good will. After he leaves, the general transforms into Matai Shang. Another grey robed man informs Matai Shang that the trio have reached the River Is more quickly than expected, and Matai Shang teleports himself to a rock formation overlooking the river.
At the river, Sola tries to embark on the journey to the Gates of Is alone, which is suicide. When Carter tells her that Tars Tarkas is her father, she tells him she wants to honor her father by dying. Carter convinces Sola to accompany him and Dejah, then decide what honoring her father means. Eventually the trio arrive at a giant umbrella-like rock formation, the Gates of Is. Carter grabs Dejah and jumps to the top of the rock formation. When they began to walk around the top, the center begins to lower, so a set of stairs emerges. The stairs lead into a cave. When Dejah touches the medallion to the ground, the floor turns into the same webbed blue material that forms Sab Than’s weapon. The pair discover the same glowing nine-legged spider symbol on Carter’s gold, and Dejah identifies the symbol as the nine rays. When they place the medallion on the group, the solar system appears on the ground, which each planet following its yearly rotation around the sun. Carter steps on Jasoom/Earth, and a set of symbols connects the planet to Barsoom. Dejah tells Carter she could decipher the symbols if she had the correct codes, but those scrolls are still in Helium. Carter becomes angry, thinking that Dejah is still trying to convince him to go to Helium, but Sola’s distressed cries distract them. Outside, a different tribe of Tharks, led by Matai Shang, are shooting at Sola. Carter and Dejah jump down to the boat, and the trio paddles back to where they left Woola.
They set out on their horse creatures, still pursued by the murderous Thark tribe. Eventually they near Helium, but the Tharks are approaching too quickly for all of them to reach the city in time. Carter flashes back to coming home to the ravaged cabin in which he lived with his wife and his daughter. Their burned bodies lie inside the doorway. He tells Sola to take Dejah to the safety of Helium, while he stays to fight the Tharks. He tells them that he has already been too late before. Dejah protests, but Sola grabs her and runs. Carter tries to force Woola to flee with the women, but faithful Woola remains anchored to his master’s side. Carter, who took Dejah’s sword, flashes back to burying his wife and daughter. The Tharks reach him, and each slash of his sword coincides with his shovel (on Earth) digging into the ground. He and Woola tear into the Thark tribe as he flashes back to placing his wife’s wedding ring on his hand. The Tharks begin to overwhelm him, and Carter disappears under a pile of alien warriors.
Dejah reaches Helium Sola slips away, reluctant to go into a human city and is greeted like the princess she is. Her father greets her joyfully, but their reunion takes a turn for the uncomfortable when Sab Than appears. Apparently Tardos Mors was won over when Sab Than came to Helium without an escort. Sab Than tries to suck up to Dejah and even offers her his sword to kill him. Dejah considers cutting his head off but eventually agrees to marry Sab Than for the greater good of her people. She does ask for Carter to receive medical help. She and her personnel find Carter buried under a pile of dead Tharks, but his is still alive.
Carter awakens in a guarded cell in Zodanga. Dejah and her retinue have migrated there for the wedding. Red Men guard Carter, and they are reluctant to allow Kantos Kan to pay the him a visit. Once inside Carter’s cell, Kantos Kan whispers to Carter to take him hostage. When Carter is confused by his request, Kantos Kan grabs his own sword, holds it to his neck, and grabs Carter’s arm to make it seem like the latter has taken him hostage. They escape the cell, and Kantos Kan tells Carter to jump them to Sab Than’s palace. Carter doesn’t know if he can make the long jump, but he goes just far enough to grab a stone ledge and tumble into Dejah’s bridal suite. Several servant women and an older noblewoman attend Dejah, who commands them, and Kantos Kan, to leave her alone with Carter. Dejah tells Carter that she will marry Sab Than to prevent him from destroying Helium unless Carter will help her people. Again, he declines. Dejah asks if there’s nothing on Barsoom worth fighting for, and she clearly means herself. Carter glances at his wedding rings and says no. But he’s conflicted about his feelings for the princess. Dejah then reveals that she understood the symbols at the Gates of Is. She hands Carter the medallion and tells him the phrase that, if said while holding the medallion, will take him home. Carter repeats the first part of the phrase, but he has trouble saying, “Jasoom.” Dejah urges him to say it, but just as he is about to say, “Jasoom,” Sab Than and his men burst into the room. Sab Than asks whether Dejah is alone, and when she turns to see Carter has teleported back to Earth, she says, “Yes. I am alone.” Sab Than leads her to their wedding procession, and the noblewoman attendant takes once last glance at the room before shutting the door behind her. Carter, who did not teleport back to Earth but rather hid in an alcove, goes into the hallway. The noblewoman attendant waits there for him. She transforms into Matai Shang, and when Matai Shang touches Carter, tattooed chains appear on the latter’s skin. He also takes Carter’s medallion.
Matai Shang and Carter ride through the city in a lizard-horse drawn carriage until the wedding procession halts them. Matai Shang transforms into the Red Man general from earlier and leads a now powerless Carter through the city. Carter believes Matai Shang is a Thern, but Matai Shang explains that he and his brethren oversee the rise and fall of planets. They are immortal, and they feed off of the destruction of planets. Matai Shang transforms into an old woman, and he and Carter watch Dejah’s and Sab Than’s lavish wedding procession. Dejah, obviously heartbroken, stares straight ahead and does not see Carter. Matai Shang tells Carter that that night, during the wedding ceremony, Sab Than will kill Dejah, thereby sealing his rule over Barsoom. This will presumably lead to the eventual end of life on Barsoom, and nature will follow its intended course, which is what Matai Shang and his brethren want. Matai Shang transforms into the general again and takes Carter to a flying ship. Before he can place Carter on the ship, though, Woola arrives on the scene. He attacks a shocked Matai Shang, allowing Carter (and Woola) to grab the medallion and make a getaway on the flying ship.
Carter crash-lands the ship in the desert, and Sola finds him and his faithful lizard-dog. Carter convinces Sola that they must return to her tribe to rally their forces and save Dejah. When they reach Sola’s city, however, they find that Tal Hajus has managed to overthrow Tars Tarkas. Carter is thrown into prison to await a turn in the fighting pits, and though Sola will not fight in the pits, she will also be executed. A wounded Tars Tarkas greets Carter in the Thark prison. He becomes enraged when he discovers that Sola did not die on the River Is but will instead be executed in a much crueler manner. He and Carter emerge into the battle arena, littered with the carcasses of small boar-like creatures that Tars Tarkas killed. Tharks chain Carter to a rock, and Tal Hajus releases a white ape, a massive blind ape-like brute with multiple arms. Tars Tarkas initially seems to weak to fight, but when he and Carter team up, they begin to overpower the white ape. Tal Hajus releases a second white ape, and Sola, who is under Sarkoja’s watch, screams out, “Father!” Sarkoja tries to force Sola to watch her father’s death, but Sola attacks her, and both Tharks fall into the arena. A white ape quickly eats Sarkoja, but then Carter manages to kill it. He kills the second white ape by raising his sword just as the creature leaps upon him, Sola, and Tars Tarkas. He cuts his way through the white ape’s body and emerges victorious, with Sola and Tars Tarkas, through its back. Carter demands to fight Tal Hajus for leadership of the Thark tribe. He easily defeats Tal Hajus. Carter then tells the Tharks that if Helium falls, so will Barsoom. His speech convinces the Tharks to join him in disrupting the wedding.
Riding their lizard-horses, and led by John Carter, the Tharks burst into Zodanga but find the city is deserted. A frightened soldier tells Carter that the wedding party actually went back to Helium for the ceremony. Sab Than’s army has gone there. Carter tries to convince the Tharks to use Zodanga’s flying ships to reach Helium in time. Tars Tarkas refuses, telling Carter that Tharks do not fly. Carter takes a one-man ship and flies off towards Helium.
In Helium’s palace, a priestess leads Dejah and Sab Than through the wedding ceremony. When Barsoom’s two moons align, their light flows into the palace through a giant monocle. It illuminates the wedding party. Sab Than and Dejah must drink from the same chalice before they truly wed, but Carter crashes through the palace roof before Dejah can take her sip. Sab Than tries to kill Dejah, but Carter prevents him. The wedding ceremony devolves into a chaotic fight between Red and Blue Men; when Sab Than reverses the monocle, the beam of light shoots into the night sky, and his awaiting army invades Helium. Just when all seems lost for Carter and the Blue Men, the Tharks arrive in flying ships and turn the battle’s tides. Matai Shang runs through the battle, trying to recapture Carter’s medallion. He morphs into Dejah, but Carter sees through his disguise and knocks him off a ledge. Matai Shang shoots the blue webbed material at Carter and Sab Than. The blue webbed material constricts and kills Sab Than, but Woola again attacks Matai Shang, breaking the material’s hold on Carter. Tars Tarkas gets ahold of the medallion, and Matai Shang morphs into Carter in order to get the medallion. But the real Carter stops him, and Matai Shang teleports away.
When Helium has won the battle, Carter and Dejah embrace. Carter takes his wife’s wedding ring off of his finger and proposes to Dejah with it. She accepts his proposal, and they finish the ceremony that she and Sab Than began. Later, Dejah awakes in their bed and finds Carter standing on their balcony. He looks out at earth and tells her he feels like he left a light on back there. Though Dejah is concerned, he kisses her and tells her to go back to bed. He will take a short walk before returning to her. He and Woola stroll to another balcony, and he tosses his medallion into a deep canyon. Carter turns to go back to Dejah, but a soldier stops to him to thank him. Woola growls at Carter, but it’s too late. The soldier morphs into Matai Shang and sends Carter back to Earth.
Carter awakens in the Evil Spider Cave. His beard is very long, and he sees that Powell’s skeletal remains at the cave’s entrance. Carter sobs and tries to return to Barsoom, but he cannot. In a voice over he tells Ned that he has spent the past eighteen years looking for another medallion. He finally found one a few months ago, but he couldn’t return to Barsoom immediately. The Therns have been following him ever since his return, and when Carter goes back to Barsoom, he will leave a replica of himself on Earth. If the Therns kill that replica, he will die on Barsoom. Carter instructs “Edgar” to watch over his body; in fact, the Therns might already be breaking into his mausoleum.
Ned rushes to his uncle’s mausoleum. He remembers that his uncle never called him “Edgar” and presses the ‘N,’ ‘E,’ and ‘D’ in the ‘Inter Mundos.’ The mausoleum door opens, and Ned is shocked to discover his uncle’s body is missing. Matai Shang appears behind him, but before the grey robed man can do any harm, John Carter appears and shoots him. “Immortal ain’t bulletproof,” he tells his nephew. He takes Matai Shang’s medallion; he faked finding another medallion to lure Matai Shang out of hiding. Carter bids Ned goodbye and urges him to do something meaningful with his life. Then he shuts the door, lays down on the body slab, and says the phrase that will return him to Barsoom.
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