Richard (Kenneth Branagh) is assigned alternative service as a consequence of a misdemeanor. A social worker connects him with the mother of a young woman, Jane (Helena Bonham Carter), who is suffering from Lou Gehrig’s disease. Feeling reluctant and unequipped for such responsibility, he starts taking her on tame outings suggested by her mother. Initially she is hostile and resistant; gradually he gives way to her insistence on unpermitted activities: he takes her on a carnival ride, drives her around in his jeep, makes her dinner at his shabby rural cottage, about all of which the mother remains clueless.

Jane acknowledges that he is the only one who treats her like an adult. In a rare moment of vulnerability, she asks him to help her lose her virginity, not necessarily to "do the awful deed" himself, but to help her hire or find someone who will give her an experience of sex before she gets to the point where it’s impossible. He refuses, she won’t see him, and for a time her mother tries to find another caregiver--a hopeless failure--a woman who talks down to her.

Richard attempts other community service and runs into comic difficulties attempting to help old women, clean toilets, and finally retreats to his outpost where he is building a plane out of scrap metal and junk in a barn. He’s insolvent, but determined to carry through his project, if only, like the Wright brothers, to keep it aloft for 12 seconds. His landlord announces that he’s selling the place and Richard and his airplane will have to clear out within a month. This impels him to try his biplane.

In the meantime, Jane searches internet dating agencies, advertising herself as a "hideously crippled woman" seeking sex, but gives it up. Missing her, Richard finally comes to her home and consents to take her to "get shagged" if she won’t blame him for any of the consequences. They go to London and seek agencies for the disabled that are willing to help her experience sex. The only positive response she encounters is at a nightclub specially for the disabled. She’s horrified.

They go upscale, to a hotel where "gigolos" might be available. Richard hilariously serves as her go-between. He finds one who, alas, charges 2000 pounds. Finally she says, "Okay, then, you’ll have to do it, Richard." This brings him to acknowledge that he’s "a cripple," meaning that he’s been impotent for some time. Instead of offering her himself, he offers to rob a bank. He doesn’t, however, go through with the robbery, but returns to take Jane home with him where she remains as she’s dying.

Ultimately, Jane and Richard both discover that love and friendship are what matter. He takes her up for the one flight his plane is capable of: a few glorious minutes over the sheepfields. The experience caps her life and seems to promise a beginning of his. She tells him, "You have a future, Richard. Either take it or switch bodies with me." She leaves him a final message on the voice machine which is the only way she can communicate, encouraging him to claim his life, and reflecting, "The only life you can have is the one that is available to you."