Lou Clark knows lots of things. She knows how many footsteps there are between the bus stop and home. She knows she likes working in The Buttered Bun tea shop and she knows she might not love her boyfriend Patrick.
What Lou doesn't know is she's about to lose her job or that knowing what's coming is what keeps her sane.
She goes to the Job Centre and, after several failed attempts, is offered a unique employment opportunity: help care for Will Traynor, a successful, wealthy, and once-active young man who was paralyzed in a motorcycle accident two years earlier.
Will Traynor knows his motorcycle accident took away his desire to live. He knows everything feels very small and rather joyless now and he knows exactly how he's going to put a stop to that.
What Will doesn't know is that Lou is about to burst into his world in a riot of colour. And neither of them knows they're going to change the other for all time.
Louisa and Will's relationship starts out rocky due to his bitterness and resentment over being disabled. Things worsen after Will's ex-girlfriend, Alicia, and best friend Rupert reveal that they are getting married. Under Louisa's care, Will gradually becomes more communicative and open-minded as they share experiences together. Louisa notices Will's scarred wrists and later overhears his mother and sister discussing how he attempted suicide shortly after Camilla refused his request to end his life through Dignitas, a Swiss-based assisted suicide organization. Horrified by his attempt, Camilla promised to honour her son's wish, but only if he agreed to live six more months. Camilla intends to prove that, in time, he will believe his life's worth living.
Louisa conceals knowing about Will and Camilla's agreement. However, she tells Treena, and together they devise ways that will help convince Will to abandon his death wish. Over the next few weeks, Will loosens up and lets Louisa shave his beard and cut his shaggy hair. Louisa begins taking Will on outings and the two grow closer.
Through their frequent talks, Louisa learns that Will has travelled extensively; his favourite place is a cafe in Paris. Noticing how limited her life is and that she has few ambitions, Will tries to motivate Louisa to change.
Louisa continues seeing her longtime boyfriend, Patrick, though they eventually break up due to her relationship with Will. Meanwhile, Louisa's father loses his job, causing more financial difficulties. Fortunately, Mr. Traynor offers Mr. Clark a position. Louisa realises that Will is trying to help her secure her freedom from her family. The two attend Alicia and Rupert's wedding where they dance and flirt. Will tells Louisa that she is the only reason he gets up in the morning.
Louisa convinces Will to go on a holiday with her, but before they can leave, Will contracts near fatal pneumonia. Louisa cancels the plans for a whirlwind trip. Instead, she takes Will to the island of Mauritius. The night before returning home, Louisa tells Will that she loves him. Will says he wants to confide something, but she admits that she already knows about his plans with Dignitas. Will says their time together has been special, but he cannot bear to live in a wheelchair. He will be following through with his plans. Angry and hurt, Louisa storms off and does not speak to him for the remainder of the trip. When they return home, Will's parents are pleasantly surprised by his good physical condition. Louisa, however, resigns as his caretaker, and they understand that Will intends to end his life.
On the night of Will's flight to Switzerland, Louisa visits him one last time. They agree that the past six months have been the best in their lives. He dies shortly after in the clinic, and it is revealed that he left Louisa a considerable inheritance, meant to continue her education and to fully experience life. The novel ends with Louisa at a cafe in Paris, reading Will's last words to her in a letter, that tell her to 'live well'.