In 2014, David Raskin, a 17-year-old high school senior and aspiring inventor portrayed by Jonny Weston, faces a challenging obstacle when he is accepted to MIT but cannot afford the tuition fees. Facing this dilemma, David discovers that his mother, Kathy Raskin, played by Amy Landecker, intends to sell their family home. Determined to find a way to fund his education, David teams up with his sister, Christina, portrayed by Virginia Gardner, and his friends, Adam Le and Quinn Goldberg (played by Allen Evangelista and Sam Lerner, respectively), to search through their father's belongings. Their father, Ben Raskin (played by Gary Weeks), an inventor, died in a tragic car accident on David's seventh birthday.
During their search, David stumbles upon an old camera featuring a video recording of his birthday party, where he makes an intriguing discovery—he sees a reflection of his 17-year-old self reaching toward a basement light switch. This revelation leads David and his friends to the basement, a place his father forbade them from entering. There, hidden beneath a trapdoor activated by the light switch, they uncover Project Almanac, the blueprint for a temporal relocation device that Ben had been developing. Inspired, the group decides to construct a functional time machine using the resources at hand.
As they set their plans in motion, they use a car battery from David's long-time crush, Jessie Pierce, who is played by Sofia Black D'Elia, to power the device. The first experiment—a seemingly innocent venture of sending a toy car back in time—results in a massive blackout across the neighborhood. However, the misadventure catches the attention of Jessie, whom they bring into their circle for the next phase of their experiment.
Excited by their success, David, Jessie, Christina, Adam, and Quinn embark on a journey back to the day before, leading to a chaotic incident where Quinn, during a moment of teenage mischief, draws a smiley face on the back of his past self's neck. This moment spirals into a dangerous feedback loop, threatening their very existence. Ignoring the risks, the group begins to use the time machine for personal gain. Adam wins the lottery, Christina confronts her bullies, and Quinn enhances his academic performance—all in the name of success.
Yet, as David tries to navigate his growing feelings for Jessie, a misstep sends him back to Lollapalooza, leading to a chain of catastrophic events that intervene in the timeline. A decision to rectify the past leads to unforeseen consequences, with future scenarios unraveling in ways David never intended, including accidents impacting his friends and other unsuspecting individuals.
Desperate to amend his errors, David time-travels again, but an accidental twist sends Jessie along for the ride, leading to heated confrontations. As her past self appears, the timeline faces another threat of erasure. With mounting pressure, David attempts to prevent the creation of the time machine altogether. In a race against time, he returns to his seventh birthday, revealing the dangers of the machine to his father, Ben. David destroys pivotal components, but in the ultimate sacrifice, he vanishes from existence.
In the aftermath, David and Christina sift through their father's belongings and discover the remnants of their father's camera, which surprisingly contains all the footage of their time-traveling escapades. The revelation becomes a bittersweet reminder of their adventures and losses. With newfound wisdom and a second chance, David approaches Jessie, about to embark on their journey again with the insight to "change the world."
