C.J. Walker is a remarkably ambitious science prodigy determined to create a time machine with her best friend, Sebastian "Bash" Thomas. However, their first attempt at this groundbreaking invention ends in failure just before the last day of school.
In their final class with Mr. Lockhart, their science teacher, C.J. and Bash hope to showcase their class project at the upcoming science expo slated for July 5. Eager to impress potential scouts, C.J. pleads with Mr. Lockhart to boost their project grade, to which he responds with a thought-provoking invitation to ponder the ethical and philosophical implications of time travel.
Their second attempt proves successful, catapulting them back in time by 24 hours. In an impulsive fit of retribution against her ex-boyfriend Jared, C.J. decides to pull a prank that spirals out of control; following her, he ends up getting hit by a car, resulting in a fractured arm. This confrontation leads to a conflict with Bash, and in the haste of their chaotic adventure, they nearly encounter their past selves before making their way back to the present.
On July 4, C.J.'s older brother Calvin and his friend Dennis leave a party after an altercation with Jared. As they walk away, they inadvertently collide with two men rushing down the street. Moments later, a police car appears, and the officers, suspecting them of being the thieves in pursuit, tragically take Calvin's life.
Following Calvin's heart-wrenching funeral, C.J. is determined to go back again to save her brother. Despite Bash's concerns about the dangers of time travel, he agrees to assist her. They orchestrate a plan to reroute Calvin and Dennis before the fatal encounter, but in a twist of fate, Jared sees them and gives chase, derailing their efforts, and they arrive too late to save Calvin.
Undeterred, they attempt a third journey, enlisting the help of their classmate Eduardo and acquiring his robust circuit boards. In this timeline, C.J. attempts to thwart a convenience store robbery, aiming to avert any police confrontation by warning the store owner. However, as past events unfold, Bash unwittingly finds himself in a perilous position, resulting in his tragic demise when he is shot. C.J. emerges outside the shop only to witness the heart-wrenching sight of current Bash dying on the sidewalk as the past version succumbs to his wounds.
Desperate to save both Bash and Calvin, C.J. embarks on a fourth journey. They manage to find Calvin and Dennis just in time, but persuading them of the bizarre reality proves challenging. When the officer who previously shot Calvin appears, Calvin protests his innocence regarding the robbery. With the situation escalating, he makes the ultimate sacrifice to ensure C.J. and Bash's escape.
Upon returning to the present, Bash confronts C.J. about her multiple travels through time. She admits that their last attempt was her fourth, during which he had died. Furious, he forbids her from trying again, but undeterred, C.J. makes a fifth trip back in time, venturing forth alone with a newfound determination. She breaks into a sprint, ready to alter the course of fate once more.
