Henry Roth is a veterinarian at Sea Life Park on the island of Oahu who has a reputation of womanizing female tourists. He shows no interest in committing to a serious relationship and his closest friends are Ula, a marijuana-smoking Islander; his assistant Alexa, whose sexuality and gender is unclear; Willy, his pet African penguin and Jocko, a walrus.
One day Henry’s boat breaks down while he is sailing around Oahu. He goes to the Hukilau Café to wait for the coastguard. There, he sees a pretty young woman named Lucy Whitmore, who makes architectural art with her waffles. Henry assumes she is a local, which prevents him from introducing himself, but the next day he comes back. Lucy and he hit it off instantly and she asks him to meet her again tomorrow morning.
When Henry goes back to the café, Lucy shows no recollection of ever meeting him. The restaurant owner Sue (Amy Hill) explains to Henry that a year ago, Lucy and her father Marlin went up to the North Shore to pick a pineapple for his birthday. On the way back, they got into a serious car accident that left Lucy with Goldfield Syndrome, a type of anterograde amnesia. She wakes up every morning thinking it is Sunday, October 13 of the last year. To save her the heartbreak of reliving the accident everyday, Marlin and Doug, Lucy’s lisping steroid-addicted brother, relive Marlin's birthday by doing numerous tasks, including putting out October 13's newspaper, watching the same Vikings game, and refilling Lucy’s shampoo bottles.
Despite Sue’s warning, Henry decides to try and get Lucy to have breakfast with him. Eventually he does, but it ends poorly when Henry accidentally hurts Lucy’s feelings. He follows her home to apologize where Marlin and Doug instruct Henry to leave Lucy alone. Henry begins concocting ways to run into Lucy through the following days such as pretending to have car trouble, creating a fake road block, or by having Ula beat him up. Eventually, Marlin and Doug figure this out due to Lucy singing The Beach Boys "Wouldn't It Be Nice" on the days when she meets Henry.
One day, as Henry is about to sit with Lucy at breakfast, she notices a police officer writing her a ticket for her expired plates. Lucy attempts to argue that they are not yet expired, and takes a newspaper to prove herself, but sees that the date on all the newspapers is not October as she thought, and Marlin and Doug are forced to admit their ruse when she confronts them.
Henry comes up with an idea to make a video explaining to Lucy her accident and their relationship and play it every morning for her. She watched the tape and is hurt, but eventually comes to her senses and she is able to spend the day by picking up where the tape says she left off. She spends more time with Henry and goes to see some of her old friends. Lucy decides to erase Henry completely from her life after learning of his decision not to take a sailing trip to Bristol Bay to study walruses, something that has been in planning mode for the past 10 years. He feels he cannot leave Lucy for the year it will take him. Henry reluctantly helps her destroy her journal entries of their relationship.
A few weeks later, Henry is preparing to leave for his sailing trip. Before he goes, Marlin tells him that Lucy is now living at the brain institute and teaching an art class. He also tells him that she sings. Then, he gives Henry a Beach Boys CD. Listening to the CD, Henry becomes emotional and curses Marlin for giving him the CD and making him feel so emotional. He then remembers that Marlin once told him that Lucy always sings after she meets him. He then realizes that Lucy remembers him and turns around to go home. She says she does not remember, but then she dreams about him every night and paints pictures of him. They reconcile.
Some time later, Lucy wakes up and plays the tape marked “Good Morning Lucy”. It again reminds her of her accident, but ends with her and Henry’s wedding. From the tape, Henry says to put a jacket on and come have breakfast when she is ready. Lucy then sees that she is on Henry’s boat, which finally made it to Alaska. She goes up on deck and meets Marlin, Henry and their young daughter, Nicole.