Seventeen-year-old Ben Marshall is the sensitive son of complacent Robert, a vicar obsessed with ornithology, and domineering Laura, who performs numerous charitable acts while ignoring her family's emotional needs, such as forcing Ben to deliver meals on wheels despite his having no car. Despite her strong religious beliefs, she is having in an affair with Peter, a young curate portraying Jesus Christ in the church pageant she is directing.
Laura refuses to allow Ben to have a mobile phone or hang around people his own age, and uses his driving lessons to be ferried around for her affair with Peter. Miserable, Ben writes poems for Sarah, a girl he knows from church. He reads her his most recent "Sarah Poem", but she rejects him.
At his mother's urging, Ben seeks summer employment to pay for the upkeep of Mr. Fincham, a mental patient Laura has adopted. Ben responds to an ad placed by Dame Evie Walton, an alcoholic, classically trained actress; reduced to a role on a daytime soap opera when her career began to fade, she has not worked since. In search of a companion to assist her in the house and drive her to various appointments, Evie immediately takes to Ben and offers him the position.
When Laura refuses to allow Ben to take a camping trip with Evie, she suggests they take a drive in the country instead, then "swallows" the car key when they find an idyllic spot for a campsite. In the morning, she announces Ben must drive her to the Edinburgh International Book Festival, where she has been invited to read poetry. Their road trip provides an epiphany for Ben, who has his first sexual experience with Bryony, one of the young women organizing the event. He learns the importance of accepting responsibility and honouring commitments, and finds the inner strength to stand up for himself.
Returning home, Ben is interrogated by Laura, who believes Evie has "corrupted" him. Evie comes to visit, but is rebuffed by Laura. Ben discovers this, and in a fit of rebellion walks offstage during the pageant, riding his bike to Evie's house where the two friends reconcile.
Evie arrives at the Pageant and, "portraying" the part of God, whips the crowd into a religious frenzy, allowing Ben to join her in the car park. Laura chases the pair and tries to weaponise the Bible once more, but Ben finally tells her to go away. Laura is run over by Mr. Fincham, whose mental state has steadily declined. When Ben visits her at hospital, she tells him her near-death experience has made her a prophet, and that God says she must divorce Robert and run off with Peter, whom the Bishop has fired; Peter takes Laura to Cornwall to convalesce.
Ben storms off, and runs into Sarah, who prattles in a condescending tone that the affair was God's will. Finally speaking his mind, Ben tells Sarah to "fuck off". On Evie's advice, Ben buys a tent and moves into the backyard. Robert tries to speaks to him, but Ben shouts that Robert, not Laura, should have asked for divorce. His father reveals that he did, explaining that he loved his wife and had tried to be faithful even when she was not. Finally free from Laura's brand of Christian fanaticism, Ben and his father reconnect.
Ben visits Evie to tell her he is moving to Edinburgh to attend university and study English. Evie is pleased, though saddened Ben will no longer be working for her. He reads her a last poem expressing his gratitude for her friendship, for which Evie compliments him. Ben promises to visit her whenever he is home from college.
The film closes as Ben, finally free, walks through the park on his way home to start packing for Edinburgh.
Julie Walters as Evie Walton
Rupert Grint as Ben Marshall
Laura Linney as Laura Marshall
Nicholas Farrell as Robert Marshall
Michelle Duncan as Bryony
Tamsin Egerton as Sarah
Oliver Milburn as Peter
Jim Norton as Mr. Fincham