A teenage boy and his girlfriend. They are happy and very in love, but tragedy strikes; the boy gets drunk at a party, drives home with her, gets in an accident, and she is killed. He feels horrible, naturally.
But the girl isn’t gone; she returns to the world as a ghost, not willing to pass on and be without her love. Unfortunately, he can neither see or hear her. She can whisper things in his ear and be perceived subconsciously, move small objects when he isn’t looking, but never make her presence really known.
For a while she is content to simply follow him around and comfort him as best she can. It even seems she may be having a positive effect; she is able to subtly influence him to quit drinking, clean up his act, and turn his life around. All seems well in a bittersweet way.
Things start to change as time passes. The boy gets older, the ghost does not. He starts to move past her death, no longer gazing longingly at the pictures of them together, not visiting her grave as much. While the ghost wants to see him happy, she is miserable that she is being forgotten, as she is unchanging and still feels love for him.
A few years pass, and the now young adult man starts dating other women. The ghost is upset and jealous. She starts to sabotage his dates, causing little accidents, using her ability to be subconsciously heard to make him say awkward things, basically being a bitch and ruining the young man’s relationships.
Unable to find happiness, the young man begins to backslide. He starts drinking again, he gets in fights, he loses his job. His life is turning to shit. Ghost girl feels terrible…until one night the man takes too many sleeping pills and almost dies. A horrible plan occurs to her; if he commits suicide then he’ll be dead too and they can finally be together.
She starts to egg him on to more and more self-destructive acts, whispering poisonous thoughts in his ear, trying to make him hate himself. He shuts out family and friends. He buys a gun and doesn’t really know why. The girl hates to do this, but it’s the only way…
Throughout the film, we’ve seen other people haunted by ghosts. At first this seems harmless; there’s a man whose father still follows him and gives advice. A mother who lost her daughter, but the girl still walks around holding her hand. The spirit of a young boy’s dog still romps behind him. These ghosts can see each other, and are the typical Pixar cast of quirky side characters.
At last the young man seems to hit rock bottom and is ready to end it all. Not wanting to watch her love die, the ghost departs to leave him to it. She heads to the park, and sees some of the other people with ghosts. But now she notices things she hasn’t before. The ghostly father’s advice, always critical, seems downright abusive, constantly berating his son for his failure and inadequacy, and the son looks miserable. The little girl’s mother can’t look at a child playing without tearing up. The ghost dog is gone, however, and the now older boy has a new pet and is happy. The girl realizes that what she is doing is wrong.
She rushes back to the man, who is moments away from shooting himself. She begs him not to go through with it, pleads and cries, but is unheard. Eventually she is just barely able to direct his attention to the neglected photos of them together as happy teenagers and he puts the gun down, picks up the phone, and calls a suicide hotline.
Man enters therapy, joins a group to quit drinking again, takes medication. It’s long and hard but he makes it. The girl is happy, but she seems to be fading slowly as the months pass. She thinks she might be finally letting go and crossing over, her reward for having saved him. Eventually she departs with a smile.
Twist ending: there was no ghost. That was his guilt and depression personified. It was always his own subconscious, the part of him that hated and blamed himself for the girl’s death. Sure, the guilt drove him to quit drinking once, but the fact that he couldn’t move on drove him into ever deeper despair. All the people with “ghosts” are the same way, haunted by their pasts. Last shot is him walking down the street, content, but as we zoom out we see just how many of the people around him have ghosts, and how widespread the problem is.
TLDR: Bittersweet ghost story is a metaphor for depression.
FantasyBoudicca 
