Emotionally withdrawn Joel Barish (Jim Carrey) and unhinged free spirit Clementine Kruczynski (Kate Winslet) strike up a relationship on aLong Island Rail Road train from Montauk, New York. They are inexplicably drawn to each other, despite their radically different personalities.
Although they apparently do not realize it at the time, Joel and Clementine are in fact former lovers, now separated after having spent two years together. After a nasty fight, Clementine hired the New York City firm Lacuna, Inc. to erase all her memories of their relationship. (The term "lacuna" means a gap or missing part; for instance, lacunar amnesia is a gap in one's memory about a specific event.) Upon discovering this, Joel is devastated and decides to undergo the procedure himself, a process that takes place while he sleeps.
Much of the film takes place in Joel's mind. As his memories are erased, Joel finds himself revisiting them in reverse. Upon seeing happier times of his relationship with Clementine from earlier in their relationship, he struggles to preserve at least some memory of her and his love for her. Despite his efforts, the memories are slowly erased, with the last memory of Clementine telling him: "Meet me in Montauk. Wikipedia.