The book concerns the rapid weight-loss of one Billy Halleck, a lawyer in an upper class town named Fairview. He is married, has a daughter, and loves his family enormously. At the start of the book, Halleck is overweight. Very overweight. However, from the very beginning, he starts losing weight.
We only pick up on the real plot after two of the most significant events have happened - the first is that Halleck has run-over and killed an old gypsy woman, mainly due to his wife picking that day to give him a hand-job in the car. The second is that the town's judge and chief of police have rallied round Halleck and let him off without any charge, which angers the old woman's father, one Taduz Lemke. (Billy initially assumes Lemke is her husband, as she herself was very old - he turns out to be 106). Lemke has touched Billy's cheek and said one word to him - "Thinner".
Billy's encounter with the gypsy keeps coming back to him every time he looks down on the scales to see that he has lost even more weight - without dieting - although his rational mind keeps the idea of a curse safely behind bars for a while. After getting a check-up with the local doctor, and making sure it wasn't cancer, Billy gets more and more concerned - especially when he finds out that the judge who let him off is becoming scaly and effectively turning into an alligator, and the chief of police now has a minefield of huge spots and sores on his face and body.
Halleck gets time off work (with option to renew, as it were), and decides to pursue Lemke and see what he can do. He follows the gypsies' trail and eventually finds them. By this time, his doctor and wife are convinced that he has a psychological problem, and are constantly trying to get him to go through tests. Billy's relationship with his wife is under great strain. Anyway, Billy confronts the gypsies and in particular, Taduz Lemke. All the gypsies regard Billy as a demon, and Gina Lemke, Susannah's grand-daughter, fires a sling-shot at Billy, the ball bearing going straight through his hand. Taduz refuses to take the curse off Billy, but Billy responds by saying that he will put a curse on the gypsies - the curse of the white man from the town (the name that Taduz has called him).
The curse takes the form of Ginelli, a friend of Billy's, and gangster. When he hears that Billy is in trouble, he sends an ex-doctor round to take care of Billy's hand, then comes round in person. When he sees the state Billy is in, he gets to work straight away, and after a few days of scare tactics (I won't detail them here as they're just so brilliant in the book)
In the end, Ginellie "persuades" the gypsies to arrange a meeting between Lemke and Halleck. This meeting goes ahead as intended, and Lemke takes the curse off - in one sense. He brings a pie, and asks Billy to drop some blood into a crack he makes in it. As soon as he does so, Billy feels the curse go... but there is a terrible price. Lemke says the curse will come back unless someone eats the pie within a couple of weeks. Billy immediately thinks of Heidi, and settles down with some planning of his own, with the idea of a nice life with just himself and his daughter living together...
After the meeting, Halleck finds that Gina Lemke has killed Ginelli, but goes home all the same and reads about the details in the paper. I won't give away the ending, but suffice to say not everything goes as planned...