NOTE: This spoiler was submitted by Jeremy

The film opens on the Mushroom Planet, where Dr. Robotnik/”Eggman” (Jim Carrey) has been stranded for a while. In his time trying to find a way back to Earth, he has attempted to make mushroom coffee and has failed miserably. He soon thinks he’s found the right way to get himself home, as he manages to create a ring portal to another world. Stepping out are three warriors, who soon fall victim to Robotnik’s series of traps. Moments later, Knuckles the echidna (Idris Elba) steps out of the portal and notices Robotnik holding Sonic’s (Ben Schwartz) quill, asking him where he got that. Robotnik happily offers to show him.

In Seattle, Sonic tries out being a superhero, calling himself “Blue Justice”. He attempts to foil a truck robbery by kicking the robbers out, but he ends up careening through the streets with little control. He finds a bag of bombs and throws them out, only for them to blow up inside a garbage truck, and another bomb that he left ends up falling into the sewers and causing an explosion that sends water bursting out. Before the truck crashes into a crowd of people, Sonic quickly dismantles the truck and flees before anyone else is hurt.

Sonic returns home to Green Hills where he still lives with the Wachowskis and Ozzie the dog. Maddie (Tika Sumpter) reminds Sonic that he agreed to go fishing with Tom (James Marsden). While out on the lake, Tom tells Sonic that he is being too reckless as a hero, and that a real hero has to take responsibility for his actions and look out for the people closest to him. Sonic takes his words to heart.

Tom and Maddie prepare to leave for Hawaii because Maddie’s sister Rachel (Natasha Rothwell) is getting married. Sonic opens a ring portal for them to go through, and he is excited to stay home with Ozzie. Sonic trashes the house while having fun, but he still uses his super-speed to clean everything up. Meanwhile, not far from home, Miles “Tails” Prower (Colleen O’Shaughnessey) emerges from his own portal, as he has been tracking Sonic and is going to warn him of impending danger.

Later that night, Sonic is watching TV with Ozzie when he hears something at the door. He finds Robotnik having broken in, returning to Earth through the ring portal. Before Sonic can deliver a whooping to Robotnik, the mad scientist sends Knuckles after Sonic. The echidna uses his super strength to punch Sonic through the walls. Sonic recognizes Knuckles’s species and believes that Knuckles wants his power, but Knuckles is simply out for revenge against Sonic because the rest of his echidna tribe had fallen in their search for Sonic, as well as the knowledge to the location of an item called the Master Emerald. Before Knuckles can kill Sonic, Tails drives a cop car to pick up Sonic and uses his gadgets to drive it. Tails explains to Sonic that ever since the energy surge he caused (in the first movie), he has been tracking Sonic from his home planet and watched as he defeated Robotnik the first time, but he observed Robotnik and Knuckles teaming up, so he came to warn him. Knuckles chases after them, but Sonic and Tails manage to lose him when Tails uses his…tails…like a helicopter to fly himself and Sonic away. Robotnik helps Knuckles back up and offers to form an alliance so that they can find the Master Emerald together and kill Sonic.

Agent Stone (Lee Majdoub) is working as a barista in his own coffee shop, where he still misses his boss. Once Robotnik contacts him, he shuts down the coffee shop and excitedly celebrates the mad doctor’s return. Stone then unveils his own creations hidden in the shop and uses it to summon a pod from space to bring a horde of Robotnik’s drones down to Earth. Robotnik and Knuckles arrive at the shop and have Stone utilize his tech to help them track Sonic.

Sonic and Tails go to the home of Wade (Adam Pally) as they try to figure out how to get to the Master Emerald before Robotnik and Knuckles. Sonic still has a map given to him by Longclaw (Donna Jay Fulks), and when he touches the symbol, it activates a hologram message from Longclaw herself. She explains that the Master Emerald is capable of converting thoughts to energy, and it is hidden somewhere on Earth. She ends the message reassuring Sonic of her love for him. Determining that the compass that will help them track the Emerald is in Siberia, Sonic opens a ring portal and convinces Tails to join him on this new quest.

In Hawaii, Tom and Maddie meet Rachel’s fiance, Randall (Shemar Moore). Rachel appears to want to bury the hatchet with Tom, but she quietly warns him she will destroy him if he does anything to ruin the wedding. Tom watches Randall’s buddies and tells Maddie that he wishes Sonic had a friend group (or “squad”) like that.

In Siberia, Sonic and Tails arrive at a bar. Tails uses his tech to try and translate his words, but he accidentally ends up insulting the barmaid and other patrons. When their disguises are removed, the patrons see them as freaks and try to toss them into a fire until Tails finds a word that calls out for a dance challenge. The lead dancer shows off to a old Siberian folk song, but Sonic changes it up by playing “Uptown Funk” and having Tails join him in a true dance battle, allowing them to win over the patrons when they defeat the lead dancer. Sonic and Tails then rest up for the night, with Tails feeling proud that he has befriended his idol.

The next morning, Sonic and Tails fly over to a cave where they have tracked the compass. They see a statue of an owl warrior. Tails says that the owl and echidna tribes were enemies for centuries. The echidnas forged the Chaos Emeralds and fused them to create the Master Emerald, which the owls took away from them for fear that anyone possessing the Master Emerald might use its power to take control over everything. The two use the symbols on the map to get the compass out. Unfortunately, Robotnik and Knuckles arrive and attack. Taking the compass, Sonic snowboards out of the cave while Tails tries to fly out as Robotnik sends his drones after them, and Knuckles chases Sonic down the mountain. Sonic destroys the drones but is knocked over a ledge by Knuckles, causing him to lose his rings. While Sonic hangs on, Knuckles tells his own backstory – he was a child when his father went off with the rest of the echidnas to fight Longclaw, a battle that left no survivors, including Longclaw and Knuckles’s father. It left Knuckles with resentment and a desire to avenge his father and tribe. Sonic tells Knuckles that they both lost everything that day. Tails traps Knuckles, but when Sonic throws him the compass, Robotnik catches it and knocks Tails out. Sonic goes to help his friend, which Knuckles takes note of since Sonic chose his friend over getting the compass. Knuckles and Robotnik escape just as an avalanche occurs.

Sonic rides down the mountain while carrying Tails and tries to contact Tom, just as Rachel and Randall are set to be married. Tom goes to answer the phone and sees that Sonic is in danger, so the hedgehog asks Tom to use the ring that he gave him to open a portal and get Sonic out. The problem is, Tom accidentally switched the portal ring with Rachel’s wedding ring. He goes to get the ring before Randall puts it on Rachel’s finger and uses it to open the portal to Siberia, allowing Sonic, Tails, and a gigantic pile of snow to fly out and cover everyone. Just as Sonic tries to explain the situation to Tom and Maddie, it is revealed that Randall and all the wedding guests are agents with a corporation called GUN (Guardian Units of Nations), led by Commander Walters (Tom Butler). They capture Sonic, Tails, and Tom, while Rachel is furious to learn that her whole wedding was a cover-up.

Maddie and Rachel get some of Tails’s tech and get past the agents so that Maddie can free Tom and the boys while Rachel goes to confront Randall. He admits that while the wedding was a sham, he truly did end up falling in love with her, and she forgives him as they kiss. Meanwhile, Maddie frees her husband and friends. Just as they go outside, they see a large green beam, as Robotnik and Knuckles have found the island where the compass brought them, and they are closer to finding the temple containing the Master Emerald. Sonic chooses to go after the villains by himself because Tails got hurt and he doesn’t want Tom and Maddie to get hurt either. He runs across the water, but Tails wakes up and tells Tom and Maddie they all need to help Sonic.

Sonic makes it to the temple and finds Robotnik and Knuckles. He and Knuckles have a second showdown, with Knuckles punching as hard and fast as he can, but Sonic taps into his own powers to dodge Knuckles’s attacks and lay a beatdown on him. Sonic then tries to tell Knuckles that Robotnik is trying to get the Emerald for himself, and Knuckles finds this to be true when he turns around. Robotnik betrays Knuckles and leaves him to his fate as he absorbs the Emerald and begins to harness the Chaos Energy. The temple begins to be brought down by water, with Knuckles trapped under a pillar. Sonic helps free him, and Knuckles helps Sonic out of the water before he drowns. Although Knuckles is feeling betrayed, Sonic convinces him to join forces so that they can stop Robotnik. They are picked up by Tails, coming in on a plane.

Robotnik teleports back to the coffee shop, where Wade has Stone in handcuffs after discovering his shady activity. GUN agents surround the place outside, and Robotnik uses his new powers to dismantle their tanks and other nearby vehicles as he begins to construct his own giant Eggman mecha, which is witnessed by Tom and Maddie. Tails flies himself, Sonic, and Knuckles closer to the mecha. The three join forces to come up with a plan to defeat Robotnik. Knowing that Robotnik’s weakness is Sonic himself, the hedgehog goes to taunt Robotnik while Tails flies Knuckles high enough to attack the robot. Tails opens a hole inside, and Knuckles get in, managing to punch Robotnik hard enough to send the Emerald flying out of him. However, he uses auxiliary energy to keep the mecha running. Outside, Sonic tries to get the Emerald, while Tom and Maddie drive by and save him. Robotnik causes them to crash, and when Sonic grabs the Emerald, it breaks apart into the smaller Emeralds. He tells Tom and Maddie he loves them as they prepare to be crushed by Robotnik, and with this, Sonic absorbs the Emeralds and becomes Super Sonic, a golden, powered-up version that is ten times stronger and more powerful. He puts multiple dents in Robotnik’s suit before knocking it over completely, sending Robotnik falling with it. Sonic then releases the Emeralds so that Knuckles can put it back together as the Master Emerald once again.

Sometime later, Tom and Maddie take in Tails and Knuckles, who have now become like Sonic’s brothers. They play baseball until Knuckles punches the ball too hard and sends it flying away. The three friends agree to keep the Master Emerald safe together, and Tom and Maddie take them out for ice cream.

During the credits, GUN agents survey the wreckage of the Eggman mecha, and they assume Robotnik is dead, unaware that Stone is in disguise among them trying to find his boss. Another agent tells Walters that GUN has uncovered the coordinates to an experiment hidden for over 50 years. Walters realizes with horror that this is “Project Shadow”. Cut to Shadow the Hedgehog emerging from a pod and appearing to wake up.

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Sonic has been trying to become a hero since he has been living on Earth with Tom and Maddie, but his actions are often reckless and do more damage. His nemesis, Dr. Robotnik, manages to find a way back to Earth with the help of Knuckles the echidna, who wants to kill Sonic after his tribe all died going after him and the Master Emerald, which can be used by anyone who possesses it for dark purposes. Joining Sonic in his new quest is a fox named Tails, whose double tails can be used to help him fly.

Sonic and Tails try to acquire a compass that will lead them to the Emerald, only for Robotnik and Knuckles to take it from them. The two get help from Tom and Maddie as the villains locate the Emerald, but Robotnik betrays Knuckles and absorbs the Emerald's energy, giving him an insane amount of power which he uses to build a giant Eggman robot to go after Sonic and his friends. Knuckles teams up with Sonic and Tails and fight Robotnik. Knuckles punches the Emerald out of him, and Sonic uses the energy from the Emeralds to become Super Sonic, allowing him to destroy the robot and defeat Robotnik again. Tom and Maddie take in Tails and Knuckles as well, and Sonic agrees to guard the Emerald with his new friends.

Agents believe Robotnik is dead, while also revealing that an experiment from 50 years ago has been found. It is revealed that Shadow The Hedgehog is awake and will potentially become Sonic's newest threat.