The basic story line is that a man named Ebenezer Scrooge has lived a life of solitude and misery. He is mean to other people, so they don't return kindness to him. He gets a visit from the ghost of his former business partner, who warns him that if he doesn't change his ways, he will be miserable in death as well as in life. Marley also tells Scrooge that he will be visited by three ghosts who will help him see the error of his ways.
The first ghost is the Ghost of Christmas Past, who takes Scrooge back in time to the Christmases he spent as a child and a young man. He reminds Scrooge of the happier times in his life.
The second ghost is the Ghost of Christmas Present, who shows Scrooge that other people he knows manage to be happy even though they are poor. This reminds Scrooge that he puts too much value on money and not enough on relationships.
The final ghost is the Ghost of Christmas Yet to Come, who shows Scrooge that he will die alone and no one will mourn him if he doesn't start being kinder to people.
In the end, Scrooge reforms himself, and he reaches out to the people in his life, becoming generous instead of stingy and kind instead of cruel. See the section on the story on enotes for a more detailed summary.