The film opens with 13-year-old Sam Cleary (Javon Walton) telling the story of two superpowered twin brothers. As children, they were bullied and called freaks for their super strength, as they had hurt people unintentionally. People in town set fire to the boys’ home, killing their parents in the process. One brother grew up to become the hero Samaritan, while the other became a revenge-drive villain called Nemesis. One day, Nemesis forged a powerful hammer and lured Samaritan to a power plant where they engaged in an infamous battle where both of them seemingly died. However, Sam believes that Samaritan is still out there.

Sam lives in Granite City with his mother, Tiffany (Dascha Polanco). The two are struggling with money, so Sam joins his friend Jace (Abraham Clinkscales) to steal copper wire from an abandoned building and sell off to some guy where they make only $40. They are approached by a street gang working for local crime boss Cyrus (Pilou Absaek). One of the gang members, Reza (Moises Arias), offers the boys a chance to make real money by causing a distraction while they rob a local convenience store. Sam volunteers.

The group goes to the store where Sam makes a mess in the store pretending to have gotten hit by a car while the gang members steal what they think are lottery ticket boxes. When they get back to Cyrus, his goons see that they’re just bags of potato chips. Reza tries to lie and blame Sam, but Sam blurts out the truth. Before Reza can antagonize him, Cyrus and his right-hand woman Sil (Sophia Tatum), show up, with Cyrus complimenting Sam over Reza. Sam also sees that Cyrus is a follower of Nemesis, as he believed that he fought for people in his own way.

Sam returns home with his cut of money and gives it to Tiffany so she can give it to the landlord to let them keep staying in their apartment. Tiffany knows that Sam got it through dishonest means, and warns him about doing so. Meanwhile, Cyrus and his crew get their hands on some new weapons.

The next day, Reza and his buddies chase after Sam due to Reza being humiliated by Cyrus. They start to attack and beat him up, but they are spotted by Sam’s neighbor, sanitation worker Joe Smith (Sylvester Stallone). With his incredible strength, he throws the other two goons off of Sam and grabs Reza’s knife, damaging it with his bare hand and not bleeding. With this, Sam is convinced that Joe is really Samaritan.

Cyrus, Sil, and their other henchmen use their weapons to break into a warehouse, where Cyrus finds the mask of Nemesis, as well as the hammer he used to fight Samaritan. Joe appears to sense when the hammer’s power is activated.

Sam visits an author named Albert Casler (Martin Starr), who wrote a book on Samaritan and Nemesis. Sam insists that Joe is Samaritan, but Albert has been annoyed because Sam always pesters him when he thinks he has found someone who might be Samaritan. After enough badgering, Albert shows Sam his room with his own theories on the superhero and says how Nemesis’s hammer was found at the site of the power plant, believing that Nemesis wanted to throw the city into chaos and kill his brother, but this led to his own death.

Sam later sneaks into Joe’s apartment for more proof. He gets caught, and Tiffany scolds him, but Joe is more forgiving. They go outside, where Sam tries to get Joe to admit to who he really is, but Joe is not having it. Reza and his goons then drive their car to hit Joe as payback for him beating them up. Sam runs to Joe, who looks badly injured, but then appears to quickly heal his bones. Sam goes with Joe back inside his apartment where he continues to deny that he is Samaritan, and he eats loads of ice cream to keep his body cooled down.

Cyrus and his crew use the hammer to begin creating destruction around the city. He dubs himself the new Nemesis and rallies several impoverished citizens to support him. Joe becomes angered at the thought of Nemesis making a comeback in the news, but he continues to hang out with Sam and show him that using his strength is not necessary for using against your enemies.

Sam rejoins Cyrus and his crew over lunch, but he becomes fearful when he sees that they took in a crooked cop and beat him up for messing with their plans. Cyrus appears to let him go, only to shoot the man in the back as he is running, which Sam witnesses. As Sam runs away, he finds Reza and his goons attacking Jace. Sam fights them off but gets himself beaten up again and sent to the hospital. Tiffany admonishes Joe when he shows up there for not defending Sam.

Cyrus’s goons follow Joe to try and beat him up, only for him to wipe the floor with all of them. In the chaos, a little girl witnesses everything and approaches Joe. One of the goons then throws a grenade at Joe, so he grabs the little girl and flips a car over to shield them from the explosion. Witnesses now also believe that Samaritan really is back. Cyrus finds out and becomes enraged since it means his plans may be stopped.

Sam goes to Joe to warn him about what Cyrus is planning to do, but Joe chooses to stay out of it. Cyrus and his crew storm Joe’s apartment and open fire, but they don’t find him. They then run to the next building to go after Sam. He gets Tiffany to run with him to the rooftop, but Sil corners them at the fire escape while the other goons surround them. Sam gets a good hit against Reza, but Cyrus orders his men to take Sam, knowing that it will lead Samaritan right to them. Joe returns and finds Tiffany wounded, and she tells him what happened.

Cyrus brings Sam to a factory where he wants to finish what Nemesis started. Joe drives a truck there and crashes through the building, also setting off a bomb that kills some of Cyrus’s crew, including Sil. Cyrus rages out and fights Joe with the hammer, where it is revealed that Joe is really NOT Samaritan, but he was actually Nemesis. A flashback shows that Nemesis actually tried to save his brother, but he fell to his death in the fire, and Joe quit being a villain for his brother’s sake. Joe fights the surviving goons before his showdown with Cyrus. Joe overpowers Cyrus and bends the hammer’s handle around Cyrus to trap him before throwing him to his death. Joe’s body overheats, so Sam sprays him with water before they run out.

Sam is in disbelief that Joe was really Nemesis, but he knows that he is a hero anyway. He goes outside to be greeted by paramedics, news crew, and other citizens. Police arrest Reza and his buddies. Sam tells the public that Samaritan is back, and he saved his life. As the crowd cheers, Sam and Joe smile at each other before Joe walks away into the night, his identity remaining anonymous.

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13-year-old Sam Cleary lives in Granite City with his mom Tiffany, and they both struggle with money. Sam joins a gang led by Cyrus but is saved from his thugs by a neighbor, Joe Smith. After witnessing Joe's impressive strength, Sam suspects that Joe is actually Samaritan, a supposedly dead superhero from years earlier that went out in a battle with his twin brother, the villain known as Nemesis, who also seemingly died.

Sam spends more time with Joe, while Cyrus, who saw Nemesis as a hero, takes on the villain's identity to cause chaos around the city. After his goons get beaten up by Joe and he uses his powers to save a little girl from an explosion, Cyrus has his men kidnap Sam to lure Samaritan to them.

Joe arrives to save Sam, but he reveals that he was never Samaritan, but he was actually Nemesis. He tried to save his brother but failed, and so he gave up his villainy to do better as his brother would have. Joe fights Cyrus and throws him to his death before Sam helps him get out of there. Cyrus's surviving goons are arrested while Sam tells the public that Samaritan is back for real, as he truly sees Joe as a hero.