Luke Malloy, Jansen Panettiere, and his friends, A.J. Perkins, Jon Kent Ethridge, and Riley Johnson, Eli Vargas, perform together as a band named Steel Monkey while they head into the two-day countdown to middle school. Luke's older sister Diana Malloy, Alexandra Krosney, warns him about the dangers of junior high—especially the chance that his friends might be left behind for the cooler crowd—and the intimidating bully Meat, [Daniel Samonas](/actor/daniel-samonas, who taunts incoming sixth graders. Luke dismisses the warnings and the boys spend the night in a tent at A.J.'s house.
The following morning, the quartet visits a carnival where Steel Monkey is slated to perform in a talent show. Luke embarrasses himself in front of his crush Alice. They encounter Meat and his friends, who threaten them, and later visit a skate park where experienced skateboarder Snake, Brendan Miller, offers skating advice. Steel Monkey feels the pressure about the show, fearing their reputation in middle school could hinge on the upcoming performance.
Luke wishes the day would never end and that he could do everything during his last day of summer. Moments later, just before their onstage moment, he is knocked unconscious by a large wooden board while returning a Frisbee. He wakes in the same tent, realizing his wish has sent him back in time. He repeats the day, trying to impress Alice, teach kindness to Meat, and impress Snake, but fails each time. He tries to avoid the board, but is knocked out again, and is sent back to the morning before Steel Monkey's performance on repeat — every day, through unforeseeable means like a football, a skateboard, or even a falling meteor, and never makes it to the talent show.
As the loop stretches into weeks, Luke grows weary of the carnival cycle and spends more time at the skate park, hoping to join a crew known as The Pound. He faces a challenge against a teen named Gus, who is also trying out for The Pound. Luke constantly rewinds with embarrassment, since the rival's skating level is far beyond him. Eventually he returns to the carnival with A.J. and Riley, encounters Meat again, and tries various approaches to handle him. At one point, he even plots revenge by tying an industrial cord from a porta-potty to a departing truck.
Worried about Diana's warning and fearing he may be left behind, Luke shifts his focus away from his friends and toward the skate park, where the so‑called "cool" crowd hangs out. He begins to ditch A.J. and Riley each morning and they later catch up with him at the park.
After many cycles, Luke finally defeats Gus at the Pound tryout and is stunned when he abandons his old friends to join the group. Diana steps in and tells Snake that Luke is too young to join, but Snake insists that Luke can never be too young for the three N's: hangN, rideN, and partyN. Realizing The Pound is not like his real friends, Luke questions whether he’s cut out for the new crew, and returns to the carnival to find A.J. and Riley angry with him and brushing him off.
The next morning, Luke is gone when A.J. and Riley wake up in the tent. He sits on a park bench crying, and Diana appears to comfort him. Luke swears he’s been reliving the same day and convinces a skeptical Diana by taking her to the carnival and pointing out events before they happen. Diana apologizes for teasing him about friends and suggests maybe the loop will end if he changes his day for the better. When his crush Alice walks by, Diana smiles and urges him to start there, encouraging him to be ready for the upcoming school year and to feel more confident.
Luke confronts Meat again, warning him against bullying and claiming he’ll reveal Meat’s real name to the whole middle school—Melvin—if he doesn’t start being nicer. Meat agrees and leaves. Luke makes a concerted effort to connect with Mr. Molesky, a teacher he had previously failed to reach, and finally opens up to Alice, who says she’s glad they’ll be in class together.
Before the concert, Luke retrieves the Frisbee that resets the cycle, but nothing happens to him. He talks with Maxine, Bailee Madison, who remembers his advice and hints she might be part of the loop, and then decides to perform. Backstage, with Diana’s encouragement and Snake’s support, Steel Monkey takes the stage and launches into The Last Day of Summer. The crowd responds warmly as they begin their song, but the moment they finish, a chipmunk falls from above and knocks Luke out again.
Luke wakes up on the first day of school, but a head injury forces him to stay home. A.J. and Riley stop by after classes begin, and Luke looks ahead to the middle school year that starts the next day.
