The drama tells the story of John Lennon's teenage years from 1955 to 1960. John was separated from his mother, Julia Lennon, when he was five. His aunt and uncle, Mimi and George Smith, raised him like a son. John is close to his Uncle George, who dies suddenly when John is 14. John becomes curious about his mother, who has since had three daughters, one of whom was placed for adoption. He becomes obsessed with rock and roll music during a visit to Blackpool with Julia. When John is suspended from school, Julia allows him to stay at her house during the day to keep Mimi from finding out. Julia teaches John how to play the banjo. Mimi discovers their arrangement, but John refuses to go home with her and stays at Julia's. A week later, John overhears Julia and her common-law husband arguing about him, so he returns to Mimi's.
When John wants to start a rock 'n' roll band, Mimi buys him a guitar. John forms a band named the Quarrymen. At their first gig at a village fête, John meets Paul McCartney. Paul auditions and joins the band. Paul and John soon begin composing songs together. As the Quarrymen gain popularity, John meets Paul's friend, George Harrison, who becomes the band's lead guitarist.
At a birthday party Julia throws for John, he confronts her about his absent father, Alf Lennon. He wants to know why Julia gave him up. He also confronts Mimi, who says Julia cheated on Alf and did not want to stay with him. Alf had asked 5-year-old John to choose whom he wanted to live with. John initially chose his father, though he then wanted to stay with his mother. Without either parent having the time or money to legally determine custody, Mimi became John's custodian. John is upset by this revelation, and leaves in a drunken anger.
John moves out to live on his own. Over time, he, Julia, and Mimi become friendly. When Julia is fatally hit by a car, John is consumed by anger. Two years later, he goes to travel to Hamburg with his newly formed band, the Beatles. Mimi asks John to call her as soon as he arrives. The film ends with the caption, "John phoned Mimi as soon as he arrived in Hamburg...and every week thereafter for the rest of his life."
Aaron Johnson as John Lennon
Kristin Scott Thomas as Mimi Smith
Anne-Marie Duff as Julia Lennon
Thomas Sangster as Paul McCartney
Sam Bell as George Harrison
David Threlfall as George Toogood Smith
Josh Bolt as Pete Shotton
Ophelia Lovibond as Maria Kennedy
James Michael Johnson as Stan Parkes
Angelica Jopling as Julia Baird
Jessie Phoenix Jopling as Jacqueline "Jackie" Dykins
David Morrissey as Bobby Dykins
Andrew Buchan as Michael Fishwick
James Jack Bentham as Rod Davis
Jack McElhone as Eric Griffiths
Sam Wilmott as Colin Hanton
Christian Bird as Jimmy Tarbuck
Colin Tierney as Alf Lennon
Paul Ritter as John's headmaster
based on Julia Baird's biography of her half-brother, the musician John Lennon.