Eighteen-year-old Jack Polovsky lives in Maine. His ex-girlfriend Jess is pregnant. They get into a fight over whether she should have an abortion or not. He wants her to have an abortion, but she wants to give the baby to an adoptive family.
Jack names his son Socrates after his brother who died. He's just getting used to being a father, and goes to stores to buy diapers and formula. He kidnaps Socrates and takes him on road trips to his grandma Bob's house in Clifford.
One day, Bob calls for Jack's birthday. She has Alzheimer's, so he is surprised she remembers.
His friend Tommy hangs out with him while Jess is in the hospital. They play video games and spend time together.
On their adventure, Jack has conversations with Socrates about the universe and infinity (if there's meaning in life), and about the old. Socrates listens intently.
Towards the end, Jack, Tommy, and Jess steal Tommy's dad's yacht. They try to steer the yacht, but have difficulties, until Tommy does it and takes them to Clifford.
At the end, they make it to Clifford to Bob's house. Jack introduces his friends. The police come to her house and when Jack sees them, he runs. They take Socrates away.
The epilogue tells Jack and Socrates are at dinner celebrating his graduation.