Henry Roth (Adam Sandler), a womanizing marine-life veterinarian living in Hawaii, meets Lucy Whitmore (Drew Barrymore), an art teacher, in a café one morning. They hit it off and agree to meet in the café the next morning. The following day, however, Lucy claims not to know Henry. The café owner pulls Henry aside and explains that Lucy suffers from anterograde amnesia (called 'Goldfield Syndrome' in the movie) as a result of a car accident she was in a year earlier. Her condition has left her with no memory of anything between the day of the accident and the present, because she is incapable of converting short-term memories into long-term memories. At the beginning of each day, she loses all memory of the past day. She innocently believes every day to be October 13, 2002. Her father, Marlin (Blake Clarke), and brother, Doug (Sean Astin), attempt to re-enact the activities of October 13, her father's birthday, every day, to prevent her from suffering from learning about the accident.
Realizing that he is beginning to fall in love with Lucy, Henry sheds his philandering ways and begins devising new ways to ‘meet’ her again every day, hoping that one day she will retain her memories – and feelings – for him. Henry and Lucy’s family eventually realize that they can’t lie to her about what has happened forever, and so instead begin to leave video tapes for her to watch each morning, explaining her situation and relationship with Henry. Over time, Lucy begins to reciprocate Henry’s feelings, even going so far as to accept his marriage proposal. One morning, however, she overhears Henry telling Marlin that he has canceled his plans to sail to the Arctic on a research expedition – something he had been planning and saving money for 10 years – in order to be with Lucy. Not wanting to be the person to hold him back, she ends the relationship and commits herself to a specialist unit in hospital, allowing herself to forget all about Henry. A heartbroken Henry, meanwhile, finally saves the money needed to begin his trip, and, after a farewell from Marlin and Doug, sets sail. Shortly after departing he realizes that Marlin had hinted that Lucy had indeed remembered Henry, and he rushes back to the hospital to see her. Upon seeing him again, Lucy explains that she still doesn’t know who Henry is, but that she has somehow remembered his face and she had dreams with him in them, as evidenced by the many paintings and pictures she has produced of him whilst in hospital. Knowing that this is proof of the love she has for him, and he for her, the couple reunite.
Several years later Lucy wakes up in a strange bed with a video tape on the night stand next to her, reading "Good Morning, Lucy". She watches it, and cries as she relives her accident while the tape explains everything that has happened, including scenes from Henry and Lucy's wedding. She then looks out the window and is shocked to find out that she's on a boat in the Arctic. She comes out onto the deck of the boat to be greeted by Henry as well as her father and her young daughter sailing on Henry's ship, his dream finally fulfilled.