NOTE: This spoiler was sent in by RMiller who tells us... "The title of the film comes from a line in a poem from over 200 years ago"
How happy is the blameless vestal's lot!
The world forgetting, by the world forgot.
Eternal sunshine of the spotless mind!
Each pray'r accepted, and each wish resign'd.
Alexander Pope, "Eloisa to Abelard"
The movie begins on Valentine's Day, 2004, with Joel (Jim Carrey) waking up, and getting ready to go to work. He gets dressed, and goes outside and notices that his car has a large dent in the driver's side door. Assuming that the car next to him hit it, he inspects both cars, and leaves a note stating "thanks a lot". We see that Joel is morose, depressed, and looks down in spirit. We hear in a voice over by Joel how Valentine's Day is, in his opinion, a commercial holiday created to make people that are single
While waiting for his train to get to work, he suddenly decides to take the train going to Montauk, a seaside park area. He runs downstairs and wedges himself in the train going there. He calls his work and states he has a stomach flu or something, and that he's taking the day off.
While at the park, he walks around, cursing himself for deciding to visit the beach while it's snowing and cold. He sees another woman on the beach, and avoids eye contact. In voice over, we hear him describe how he always avoids eye contact with the opposite sex, and that substantially reduces his chances of ever meeting anyone. He sits on the beach and writes in his journal, but notes that there are no entries for months, which strikes him as odd.
At the beach, he sees a woman in an orange sweatshirt, and she sees him. Her hair is blue. She waves at him while on the return train stand, but he again avoids eye contact.
While on the train, she sees him again, and this time approaches him, and introduces herself as Clementine (Kate Winslet). She askes him not to joke about her name, but he doesn't know any songs with Clementine in it, or any jokes about her name. She says he looks vaguely familiar, and asks if he has gone into Barnes & Noble, where she works. As they strike up a clever conversation about hair color, cartoons, future plans, she joins the seat next to him, and upon arrival, they leave the train. While driving away in his dented car, he sees her on the street, and invites her to let him give her a ride to her house. She invites him in for a drink, and he recoils in shyness, causing her to become embarassed, but then he ends up in her apartment, which is wildly decorated. For example, he makes note of a collection of potatoes dressed as people from different professions are the theme on one table in her house.
After they drink, she jokes about them getting married, and she asks him, on a whim, to drive with her to the Charles River, which is frozen, and go out on the ice with her. They end up going there, and laying on the ice, watching the stars. He mentions that he feels happy.
As the sun comes up, they are driving back, and when they get to her house, she asks if she can go with him back to his place, to sleep, and asks for him to wait while she gets a toothbrush. While she's inside getting ready, he is waiting in the car, noticeably happier, and a young man named Patrick,(Elijah Wood), comes up and knocks on the window. With some urgency, he asks what Joel is doing there. Joel asks, "Do I know you? The young man backs off, suddenly.
At this point the film moves to an earlier point in time, and we realize that the film started about 75% of the way through the story, (and isn't near done jumping around yet).
We flash to a scene of Joel crying and sobbing in his car, uncontrollably. We then see Joel asking his married friends why Clementine didn't recognize him, and we can see he is still distraught over it. The film flashes back to earlier in the day, when he was in Barnes & Noble and she acted like she didn't know him, and was kissing someone else, which upset him. His friends finally reluctantly reveal that they were sent a form letter, actually a pre-printed, fill in the blanks card, that reads:
"Clementine Kruczynski has hired the services of Lacuna Inc. to erase the memory of JOEL BARISH from her mind. Please refrain from making any mention that may disrupt this process." As Joel reads the card, the name of Clementine disappears, as though written in disappearing ink.
Joel immediately finds Lacuna and makes a visit in person, demanding to know what happens. The receptionist Mary (Kirsten Dunst), is answering the phone and advising that their Valentine's Day special has ended to a caller, and tells Joel to have a seat. Joel ends up speaking to Dr. Howard Mierzwiak (Tom Wilkinson), who explains that although patient information is confidential, Clementine hired them to erase her memories of Joel.
He decides that he wants the same thing done to him, to erase Joel's memories of Clementine. "Is there any danger of brain damage?" Joel asks, "Well," the doctor states, "technically speaking, the procedure is brain damage." He assures Joel that he won't suffer anything more than he would after a night of heavy drinking. They have Joel make an audiotape declaring his intent to go through with the process, and
They begin the process, and Dr. Mierzwiak asks Joel to collect everything that he has that reminds him of Clementine, and bring it to Lacuna. They begin by placing Joel in what looks like a 50's hairdryer, and as Joel looks at items, such as the potato people, that remind him of Clementine, they measure the areas of brain activity on a monitor in the room. They ask Joel to start with his last, or most recent, memory of Clementine, which was the incident where he saw her in Barnes & Noble, but she didn't recognize him. We then flash back to their breakup. We see that Clementine came home at 3am, and told Joel that she put a big dent in the car when she came home. She also floated the idea that she was unfaithful, and was accused of Joel of having had sex, and of sleeping with people to get them to like her. Clementine storms out, and Joel sees that Clementine hit a fire hydrant on the driver's side door, and starts the car and chases her, but to no avail. From there, the doctor wakes up Joel, and tells Joel that he will be put under and will wake up in his own bed, with no memory of Clementine. Joel next is shown in bed back in his house, and he's hooked up to a computer, at which two technicians (Patrick [Elijah Wood], and Stan [Mark Ruffalo]). They end up drinking, and invite Mary, who Mark Ruffalo is trying to know better, over.
Patrick reveals that he's been romancing Clementine, and we see later in the film that he's been using the notes from Joel's process to do so. We see Clementine invite Patrick to go to the frozen Charles river and lay out on the ice, and see Patrick mimic Joel's words to Clementine. Joel's Valentine's Day gift that was never given is used by Patrick to impress Clementine, and she notes that it's like he knew exactly what she liked.
While Joel is under, we see his memories being erased. Titles from books and signs erase, backgrounds get fuzzy, and faces are blank and distorted, along with voices. Things disappear at will within the memory. The memories show that Joel met Clementine through his married friends, at the same beach that was in the beginning of the movie. We see a major fight that Clementine and Joel had over having children, in a swap meet, and we see the gift that Joel bought for Valentine's day. We also see, in various ways, snippets from their relationship. We see Clementine's hair go from Orange, which she dyes to match the sweatshirt she was wearing when Joel first noticed and met her, to bright red, to green, to blue. Joel asks why Clementine erased him from her memory, and she mentions that she's always been impulsive, and that she was hurt. After a touching memory where Joel and Clementine are in bed together, and are speaking to each other of future plans, he decides that he doesn't want to go through with the process, and would rather have the happy times intact. Caught up in memories, he tries to get Clementine to stop leaving him, but with no success. He can hear the technicians, but cannot see them. He tries to wake himself up, and during a memory in the woods, enlist the version of Clementine in his own head to help him. He decides that he may be most successful if he hides Clementine in old memories, so we see him and Clementine in the sink while he is a baby, and Clementine as one of his mother's friends, and Clementine as a young girl who protects Joel while being beat up as childhood bullies.
As memories appear where they are not supposed to be, Mark Ruffalo freaks out (after drinking, smoking pot, and sleeping with Mary), and calls Dr. Mierzwiak, who comes over late at night and fixes the problem. They are now back on track to erase memories. Eventually, he returns to earlier memories of the first day that Joel and Clementine met, and the house on the beach where they broke into and fell for each other that first day. The house begins to fall apart as the memory is erased, and Joel and Clementine try to make this last memory last as long as they can, but to no avail. Dr. Mierzwiak, after trying to leave once, has a drunk Mary attack him and try to kiss him. Unfortunately, his wife, who followed him, witnesses them kissing, and when they come out to explain (after Stan honks the horn to alert them), Mrs. Mierzwiak reveals to Mary that she's already slept with Dr. Mierzwiak. He explains to Mary that they agreed to erase her memory.
Mary, upset, goes back to Lacuna, and listens to her own audiotape, which she made before having her memory erased. She then takes boxes of patient files out of the office, putting them in her car, and quits, even while Stan follows her and begs her to stay.
We then see Clementine and Joel in Clementine's apartment. While getting ready to go to Joel's place, as in the beginning of the film, Clementine checks her mail. She sees a letter from Mary, and her file and audiotape. Mary has written to each patient, describing what happened to them. Clementine plays the audiotape in the car, and they both become very uncomfortable. Joel confronts her, asking if this is some kind of sick joke, which Clementine denies. Joel asks Clementine to get out of the car. Back at his house, we see Joel listening to his audiotape, and revewing his file of memories, and sadness returning to his face again.
Clementine goes back to her apartment, but is extremely upset and decides that she need to talk to Joel. She goes to Joel's apartment, where the door is ajar, and finds Joel sitting on the floor with one of the paintings that he had drawn of her (which I guess he missed when he was supposed to turn over any reminders of her to Lacuna, Inc.) He is playing his own audiotape which he received from Mary, and echoing throughout the apartment is the playback of him saying how awful his relationship with Clementine was. When Joel sees her, he wants to turn off the audiotape, but she says that is only fair that she hears what he had to say, since he heard her tape earlier. So the tape plays back all the bad things that Joel thought of her at the end of their previous relationship, such as Clementine sleeping with people to get them to like her, etc. Clementine runs out of Joel's apartment, and after a moment of thought, Joel runs after her. She stops, and he asks whether they can work it out. She says something to the effect of, even if we try, I'm going to end up thinking that you're really boring, and you're going to end up thinking I'm a slut. But Joel then just looks at her straight on, and says, "Okay". So she looks at him, and says, "Okay" back, since they both realize that even though they had gone through it before, perhaps they weren't doomed to make the same mistakes and could make it work out this time around, now knowing what the pitfalls are.
So this movie actually ends on a somewhat optimistic note.