Come on, you know the story.
A town of merry Christmas-lovers called the Whos goes about their annual holiday preparations. Up in the caves north of their town, though, lives a character named the Grinch who absolutely hates Christmas. We're not really sure why (who could hate presents and feasting?), but hey, hatred never really had a good explanation.
At any rate, the Grinch has had enough of this Christmas business and he sets about trying to ruin it for the Whos. He sneaks into all of their homes and steals their Christmas decorations, meals, and gifts—an act that is both illegal and extremely Grinchy. He then takes his loot up to Mt. Crumpit and prepares to throw it off the side of the mountain. Harsh.
At the last minute, though, the Grinch hears a sound coming from Who-ville—the sound of all the Whos joining together in song despite the fact that their homes have been ransacked and their gifts are all gone. Finally, the Grinch feels a twinge of remorse (and maybe even Christmas spirit?), so he brings back all the things that he stole. He arrives in time to join in on the Christmas festivities.
Now, with that overview out of the way, let's look into the details of the Grinch's devious plotting and his 180-degree turn around.