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Adam (Joseph Gordon-Levitt) is a young man who has a girlfriend, Rachael (Bryce Dallas Howard), despite Adam's best friend and co-worker Kyle's (Seth Rogen) disliking her. One day, Adam decides to see a doctor because of back pains and discovers that he actually has a rare type of cancer and must undergo chemotherapy in order to reduce the tumor. The doctor gives Adam the phone number of a hospital therapist in case he needs someone with whom to talk.

Adam tells Rachael about the cancer and tells her that he won't mind if she prefers to leave him. She says she'll stay by his side. However, she eventually starts to display growing signs of detachment to him. Adam also tells his mother Diane (Angelica Huston), who is also taking care of Adam's father, Richard (Serge Houde), who has Alzheimer's disease; and Kyle, who worries for his friend's survival.

Adam starts seeing an inexperienced psychologist, Katherine "Katie" McCay (Anna Kendrick), who helps him deal with his problems. He also befriends Alan (Philip Baker Hall) and Mitch (Matt Frewer), two older chemo patients receiving treatment alongside him, and grows fond of Skeletor, an old dog which Rachael rescued from the pound.

Kyle starts using Adam's illness to get dates, which annoys Adam. On one of these dates, he sees Rachael kissing another man and takes a picture of her affair on his cellphone, which he immediately shows to Adam. Angry, Adam tells Rachael to leave.

Adam and Kyle hang out with Alan and Mitch, and Adam becomes impressed with Mitch's loving relationship with his wife. Later on, Kyle urges Adam to use his cancer to get dates, but Adam doesn't want to do that. Adam encounters Katie who gives him a ride home. Adam starts developing an attraction to her. The next day, he and Kyle go to a bar, where they meet two women. Adam has sex with one of them, but is unable to enjoy it due to his back pains.

The following day, Adam goes to chemo, where he is told by Alan that Mitch died the previous night, something which shocks and leads him to believe that he has no chance of survival. After Mitch's funeral, Adam sees Katie, who advises him to stop ignoring his mother, who is already having a hard time taking care of his father without having to worry about Adam not returning her calls. Adam sees the error of his ways and reconnects with his parents, who drive him to a doctor's appointment in which Adam is informed that his tumor has grown and he'll need to undergo surgery in a few days in order to have a chance of making a full recovery.

Increasingly nervous about the procedure, Adam decides to go drinking with Kyle, but ends up lashing out at Kyle for being more interested in having sex with random women than being there for him. He proceeds to have something of a nervous breakdown and decides to call Rachael, but ends up calling Katie, who tells him they might have something after his surgery. Adam takes Kyle back to his apartment, where he discovers that Kyle has been following the instructions of a book on how to cheer up cancer patients and realizes that Kyle was doing his best to prevent Adam from reminiscing about the cancer.

Adam undergoes surgery as Kyle, Katie, Diane and Richard await. The doctor informs them that, although the journey will be long, Adam will survive.