Will Traynor was a successful, wealthy man who enjoyed all aspects of his life until being paralyzed in an accident.
Two years later, 26-year-old Louisa Clark loses her job at a local café. Unambitious and with few qualifications, Louisa lives with her working-class family. She is constantly outshone by her younger, more intelligent sister, Treena, a single mother. Louisa, whose job helped support her family, goes to the Job Centre and is offered a unique employment opportunity: help care for a disabled man. Camilla Traynor hires Louisa as caretaker for her son Will, despite her lack of experience. She believes Louisa can brighten her son Will's spirits. Louisa notices how falsely everyone behaves in Granta House, the Traynor's castle. Mr. Traynor, Will's father, who is having an affair, wants to divorce his wife.
Under Louisa's care, Will becomes more communicative and open-minded. 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, an assisted suicide organization. Horrified by his attempt to kill himself, Camilla promised to honor 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 is worth living.
Louisa conceals knowing about Will and Camilla's agreement. However, she tells Treena and together they devise ways to 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 although Will is significantly happier, she senses he will not abandon his plan. Louisa becomes overwhelmed and decides she can not participate in Will's assisted death. Mrs. Traynor recognizes the positive effect Louisa has had on Will and persuades her to come back. In a last and desperate try, Louisa tells Will’s family that she wants to take him on a trip. Meanwhile, Louisa's father loses his job, causing more financial difficulties. Fortunately, Mr. Traynor offers Mr.Clark a position.
Will and Louisa have frequent talks. He notices that she has a limited life and few ambitions. Will tries to motivate Louisa to change. She continues seeing her longtime boyfriend, Patrick, though they break up due to her relationship with Will.
With the help of the Internet and Treena, Louisa plans a fun holiday for Will on the island of Mauritius. The night before returning home, Louisa tells Will that she loves him. Will says he wants to tell her something, but she admits that she already knows about his plans with Dignitas. Will says their time together has been special, but that 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 to see him in such good physical condition. Louisa, however, abdicates as his caretaker, and they understand that Will intends to follow through with his plan.
On the night of Will's flight to Switzerland, Louisa goes to see him one last time. They both agree that the past six months have been the best in their lives. He dies shortly after in the clinic. Will leaves Louisa a considerable amount of money to continue her education and to fully experience life.