Marcus Jordan and his mother, Barbara, move from Kansas to Kennesaw, New York, before his junior year of high school. Marcus spends his summer at a public park, training for football tryouts. A man named Charlie interrupts Marcus’ practice and starts to throw the ball around with Marcus.
The impromptu workout goes well until Charlie throws the ball, and it breaks the window of a nearby, parked car. Charlie runs away, but Marcus leaves a note with his name and number so he can pay the owner back for the damage. Marcus is confused about why a grown man like Charlie would run away rather than own up to his mistakes.
Marcus goes to football tryouts and meets Troy Popovich, the quarterback, and the other members of the Raiders team. The team had a perfect season the previous year. They don’t want Marcus to come in and ruin things for them. Many on the team try to sabotage Marcus’ tryout by dropping all the passes he throws, but Coach Barker realizes what they’re doing. Despite the team’s antics, Marcus is given a spot on the Raiders team. Alyssa, the head cheerleader and Troy’s ex-girlfriend, flirts with Marcus to make Troy jealous.
Marcus is afraid of being tackled, so the next time he sees Charlie in the park, he tells the man to tackle him. Charlie hits hard, and Marcus figures if he can take a hit from a grown man, he can handle anything. Marcus is confused about why a grown man isn’t at work in the middle of the afternoon and why he’d want to play with a teenager in the park. But he appreciates the workout and football pointers Charlie provides. Marcus makes it his habit to meet Charlie in the park to workout, though Charlie is unpredictable and unreliable about showing up.
Mr. Oliver and Officer Deluca confront Marcus after one of his workouts with Charlie. Mr. Oliver owns the car that was damaged the first day Marcus and Charlie met. Marcus accidentally left an incorrect phone number in his note, and Mr. Oliver decided to involve the police. The mistake is resolved at the police station, and Barbara comes to get Marcus. He tells his mom that he was throwing the football around with a friend, but doesn’t elaborate on his friendship with Charlie.
Marcus brings a check to Mr. Oliver to pay him back for the car window, but the man is rude and doesn’t accept Marcus’ apology. Charlie witnesses the encounter and suggests that they prank Mr. Oliver. That night, they spread sugar around Mr. Oliver’s shop to attract bugs. In the morning, Marcus goes to the park and hides where he can see Mr. Oliver’s shop. Charlie arrives, and his daughter Chelsea isn’t far behind. She warns Marcus to leave her father alone. Marcus knows Chelsea from school and realizes that Charlie is also Troy’s father.
Marcus recognizes the name Charlie Popovich. An internet search reveals Charlie was a former NFL star known as the King of Pop. He got the nickname for his tackling ability. When Marcus mentions it to Troy, the quarterback warns Marcus to stay away from his dad.
Marcus continues to meet with Charlie, and Marcus takes the blame for the prank on Mr. Oliver. During one workout, Marcus dislocates his shoulder. Charlie tells Marcus how to pop it back into place. Marcus doesn’t want to follow Charlie’s instructions and insists on going to the emergency room. Charlie leaves to get his car, but he doesn’t return. Marcus ends up fixing the shoulder himself and vows to cut Charlie out of his life.
Marcus and Alyssa grow closer, and she invites him to a party after of the football games. At the party, Troy finds them making out and becomes angry. Before a fight can break out, Chelsea arrives to tell Troy that Charlie is missing. Marcus overhears and thinks Charlie might be at the park. He finds Charlie and asks him why he’s at the park at night. During the conversation, Charlie reveals that he thinks he’s three weeks younger than Marcus and that he’s forgotten the prank they pulled on Mr. Oliver.
After he sees Charlie safely home, Marcus goes home and does some research about Charlie’s memory loss and confusion. He finds an article linking multiple concussions of NFL players to early onset Alzheimer’s. All the symptoms explain Charlie’s behavior.
Officer Deluca brings Marcus into the police station again — this time for TP’ing Mr. Oliver’s shop — though he didn't. Because of the repeated offenses, Mr. Oliver has decided to press charges, and Marcus is given a date to appear in court. Marcus refuses to name Charlie as the culprit because he doesn’t want to reveal Charlie’s secret condition.
Barbara brings Marcus to the newspaper office where she works since she doesn’t trust him to be left on his own. He sees an old picture at the office with a young Charlie outside of Dingley’s Hardware Emporium. Every time Charlie pranked Mr. Oliver, he called him Dingley. Marcus realizes that Charlie is remembering the past and thinks that Mr. Oliver is Dingley. Charlie always calls Marcus "Mac." Marcus wonders if Mac is another person from Charlie’s past.
At school, Chelsea confirms that Charlie has Alzheimer’s, but she insists it isn’t anyone’s business. Marcus sees a team photo from when Charlie played football at the school. James McTavish is next to Charlie in the photo. Marcus wonders if that’s Mac.
Using a website that helps locate old classmates, Marcus tracks down James McTavish, who lives a few hours away. He goes to visit James. Marcus reveals Charlie’s condition and how he thinks Marcus is Mac. The real Mac agrees to talk to the police if it will help Marcus with the Mr. Oliver situation. He also tells Marcus that Charlie is being inducted into their old college hall of fame at the upcoming homecoming game.
When Marcus talks to Troy and Chelsea about their father’s upcoming honor, they don’t know what he’s talking about. After searching their house, they discover that Charlie has been opening his mail, not understanding what it is, and stuffing the letters in the glider for the porch swing. The family decides not to take Charlie to the ceremony. They are afraid it will upset him.
When Marcus finds out that the family isn’t taking Charlie to the ceremony, he plans to take Charlie himself. With Mac’s help, they get Charlie to the ceremony. Charlie has a great time, and Marcus believes he understands what’s going on. When Charlie goes missing, his family worries. Chelsea finds a live feed of the ceremony online and sees both Charlie and Marcus there. The police are called to bring Charlie and Marcus home. On the way back, Charlie talks about pranking Dingley, and Officer Deluca realizes that Marcus has been covering for Charlie.
They arrive back in time for Marcus to play in the Raider’s rival game. Troy is out of the game, claiming that he broke his hand. Really, he’s afraid of taking a hit and ending up like Charlie. Marcus goes in as quarterback. When he takes a hard hit and is knocked out, Troy insists that he can finish the game for Marcus. When questioned about his hand, Troy says it’s fine, and that Marcus doesn’t want to end up like Charlie. Troy plays the rest of the game cautiously so he won’t get tackled. The Raiders win the game.
With Charlie’s condition out in the public, Mr. Oliver drops his charges against Marcus. Mrs. Popovich realizes Charlie’s condition has progressed further than she wanted to admit, and she can’t care for him like he needs to be cared for. Since Charlie likes Marcus, Mrs. Popovich asks him to accompany them to the local retirement home. When Charlie realizes his family wants to put him there, he breaks down into tears. It is the first time he’s realized the gravity of his own condition.
Troy stops playing football but still brings Charlie to the games. At one of the final games of the year, Marcus sees Charlie following a hawk to the top of the bleachers. Charlie is reliving a memory from his past. Marcus tries to get Troy’s attention from the field. When that doesn’t work, he runs after Charlie. Charlie and Marcus share a look before Charlie falls to his death. Marcus is convinced that Charlie knew what was going on in his final moments.
Marcus attends Charlie’s funeral. After the service, he gives his condolences to the family. He and Troy share a handshake. The two are no longer at odds.