Henry Roth (ADAM SANDLER) is a love 'em and leave 'em kind of guy who works as a veterinarian at Hawaii's Sea Life Park. When not tending to the animals with the aid of the gender nebulous Alexa (LUSIA STRUS) or hanging out with his wild friend Ula (ROB SCHNEIDER), Henry dates many women, but doesn't want to settle down since he dreams of heading to the artic to study walruses.
That changes one day when he happens upon a diner run by waitress Sue (AMY HILL) and short order cook Nick (NEPHI POMAIKAI BROWN). There, he spots the lovely Lucy Whitmore (DREW BARRYMORE) who's building structures out of her breakfast waffles. The two immediately hit it off, but the next day she doesn't seem to recognize him. Sue then informs Henry that Lucy suffered a brain injury about a year ago that left her long term memories intact, but has made her unable to retain any short-term ones. With her physician, Dr. Keats (DAN AYKROYD), unable to help her, every day is October 13th to Lucy, no matter what she did the day before.
As a result and despite the redundancy for them, her father, Marlin (BLAKE CLARK), and steroid-using brother Doug (SEAN ASTIN) go to extreme measures to make sure she experiences every day as if it's that date. They do so to protect her and thus become worried that Henry's obvious attraction to her might upset their delicate temporal balancing act. Nevertheless, Henry falls for Lucy more each day, despite her not recognizing him 24 hours later. From that point on, he tries to find ways to remind her of him and their previous time and experiences together.