The film opens in 2050, where a pilot named Adam Reed (Ryan Reynolds) is flying a jet up in space. He is pursued by a woman named Maya Sorian (Catherine Keener), but Adam flies far enough to open a wormhole where he disappears.
2022 – 12-year-old Adam (Walker Scobell) is being chased by school bully Ray Dollarhyde (Braxton Bjerken) and his buddy Chuck (Kasra Wong). Ray beats Adam up because Adam has a propensity for making smartass comments. Adam gets suspended, and his mother Ellie (Jennifer Garner) comes in to chew out the principal.
Adam has lived with Ellie after they lost his father Louis (Mark Ruffalo) over a year earlier in a car accident. Ellie is going on a date with a coworker, and Adam cannot do anything but make snarky comments and be unsupportive towards her. After she leaves, their dog Hawking runs outside. Adam follows and finds ashes falling from the sky. He goes into his dad’s garage and finds Big Adam sitting with a bullet wound in his abdomen. When Young Adam tells his older self that he is in 2022, Big Adam realizes he made a mistake. After patching himself up inside the house, Young Adam realizes he is talking to himself as an adult. Big Adam shows Young Adam his ship that can be activated with his DNA, and he stays hidden to rest up. He tells Young Adam that once he returns to his fixed timeline, their memories will reconcile and Young Adam will most likely not remember meeting his older self.
After Ellie gets home, Adam meets her date and makes more rude comments, leading Ellie to tell her son he’s a jerk sometimes. Outside, Big Adam sees Ellie step outside, and he feels bad for her.
The next day, Big Adam goes out with his younger self where they run into Ray and Chuck again. Big Adam steps in to encourage Young Adam to go along with the fight with confidence, but Ray still beats him up. Big Adam threatens Ray bad enough to make him wet himself, and he returns to Young Adam to apologize for letting him go through that, but he tells him that they needed to let it happen, or he will not develop into his stronger self as he gets older. Young Adam looks at Big Adam’s phone and sees a photo of his wife, Laura (Zoe Saldaña), but Big Adam doesn’t want to talk about her.
Big Adam finds Ellie at a bar, where she is talking to the bartender about her issues with her son. Adam offers words of encouragement, assuring Ellie that she is doing a great job and that her son really does love him.
Adam returns home to tell his younger self that Maya, who was Louis’s business partner, is in control in the future since she exploited Louis’s discovery of time travel. Louis created a particle accelerator, “The Adam Project”, and using this, Maya has turned the planet into a dystopian hellhole. His original target was 2018, but he missed the mark. She also killed Laura when she attempted a time jump, and Adam knows that she is coming for him. Sure enough, Maya sends her goons, plus her right-hand man Christos (Alex Mallari Jr). The villains find the Adams outside, and Big Adam takes out his special weapon and fights the henchmen, who disintegrate because they died outside of their fixed time. Christos comes out to fight Adam, but they are saved by Laura appearing from the woods. After taking out more henchmen, she drives them through the woods, where Big Adam kills two more thugs.
Laura brings the Adams to a cabin to hide out. She tells them that she tried to jump to 2018 because Maya had already gone back in time to manipulate the timeline and make herself successful. After Big Adam and Laura have an intimate moment after a long time apart, she tells him that they cannot stay there, and he has to finish the mission to 2018. Adam tearfully parts with Laura again and drives his younger self to find his ship. Christos goes after them, but Laura holds him back with a landmine, though it doesn’t kill him. The Adams get in the ship and fly away, but Maya kills Laura and then goes after them. She knows that Adam only has one jump left, and there’s no other way for him to get back to his timeline. The two decide to jump to 2018.
In 2018, the Adams find Louis teaching a class. After the students leave, Louis quickly pieces together that he is talking to his son as an adult, as well as a version that is four years older than the one that he already lives with. Big Adam explains to his dad that time travel is real thanks to him, and Maya has betrayed him for her own gain. They ask for Louis’s help, but he doesn’t want to do anything else that can negatively affect the timelines. Big Adam gets angry with Louis and hits him, and Louis hits back. He tells his younger self that Louis was never a good dad and that he only cared more about his work than his own son.
The Adams go back to the cabin, where Young Adam tells his older self that he thinks Big Adam is angry with Louis because he is angry about his death, and it is easier to be mad at him than to grieve him.
Louis goes back home to Ellie and 8-year-old Adam. He wonders if Adam seems sad, but Ellie gives him reassurance. Meanwhile, Maya finds her younger self once again after previously using her when she time-jumped the first time. She warns her that Adam will be her biggest problem.
The Adams resolve to go to Maya’s company and shut the accelerator down themselves. Young Adam uses a special drone to deal with more henchmen, but Louis shows up after changing his mind, saying they can grab the hard drive that contains his algorithm for time travel. Big Adam goes inside with Louis, while Maya’s ship appears behind Young Adam.
Adam and Louis make it to the accelerator, but the Mayas show up, along with Christos and the henchmen holding Young Adam at gunpoint. At his older self’s encouragement, he causes the villains to fire a shot at the accelerator’s electromagnetic shield. The accelerator begins to destabilize, and the heroes fight the villains. Christos gets knocked into the accelerator, while Maya orders Louis to give up the hard drive. When he refuses, she fires an armor-piercing round to kill them, but the shield pulls the bullet toward the younger Maya, killing her and causing the older Maya to disintegrate and be erased from the future. The guys then flee the building as the accelerator blows up.
The three return home. Louis acknowledges that he is going to die and that it will hurt his son, but he asks them not to tell him when or how it happens. He reassures Big Adam that he is proud of him, and he hugs both versions of his son. Knowing they are set to return to their own timelines, Big Adam tells Young Adam to be a better son to Ellie. They then play catch with Louis one last time before they return.
Back in 2022, Adam starts showing more love to Ellie. Years later, Adam in college meets Laura when she walks to the wrong classroom and he offers to walk her to the right building.
