In his old age, Scrooge has become a reclusive, hoarding miser—he hates paying his clerk, he refuses to socialize with the only family member he has left, and he lives in an isolated non-residential building where he scrimps on his own food and heating. This finally comes to a head when his dead partner's ghost shows up to shake him up by threatening him with a terrible afterlife.
Mostly because Scrooge is a practical guy, he dismisses the ghost as indigestion. He is so committed to his purely rationalist worldview—no charity for those who cannot work, because that would be clouding economics with emotion; no pay for days off, because that should be considered straight-up theft; no donations to charity because the poor are themselves to blame for their own poverty—that he even manages to dismiss the ghost as a figment of his own imagination.
Ah, but dismissing the first ghost only leads to some bad mojo. Reversing the old parental chestnut about bullies—that ignoring them will make them go away—Scrooge is harassed further by other ghosts. The Ghost of Christmas Past dredges up some scenes of a once normal Scrooge as a reminder. The Ghost of Christmas Present shows him how awful life is for the Cratchit family, and how much he is missing out on by ignoring his nephew's invitations to dinner.
The last and final spirit, the Ghost of Christmas Yet to Come, scares the daylights out of Scrooge. It shows up to demonstrate that if Scrooge continues his miserable life the same way, he will end up dead, unmourned, and unremembered. In fact, Scrooge thinks he might be doomed from the start. Maybe it's too late to save himself…
… Not so. Scrooge wakes up and realizes there is still time to fix things. In the real present, it turns out that the predictions of the future were not set in stone, and so Scrooge spends the rest of his life changing his ways and becoming a better man.