Sunset Towers is a new apartment building owned by the Westing Estate that's waiting for people to live in it. Barney Northrup selects tenants for the apartments who are families, professionals, and secret-keepers: Grace, Jake, Angela, and Turtle Wexler; Mr., Madame, and Doug Hoo; George, Catherine, Chris, and Theo Theodorakis; Judge Ford, Crow, Sydelle Pulaski, and Flora Baumbach. The narrator reveals that one tenant steals, one sets off bombs, and one isn't even supposed to be there in the first place.
The tenants all move in that September. On Halloween, Turtle Wexler is dared to go into the Westing house, where she sees a dead body. The next morning, she reads the obituary for Sam Westing, of the Westing estate and Westing Paper Products, in the newspaper. Sixteen people, including most of the Sunset Towers tenants, are named as heirs in the will and are called to the reading of the will; they have to define their positions in life in order to show up. The other people called in are Otis Amber, the delivery boy; Dr. Deere, Angela Wexler's fiancé; and Sandy McSouthers, the doorman.
At the reading, led by Ed Plum, the will reveals that Westing was murdered, and one of the heirs is the culprit. The heirs are all divided into teams (eight pairs of two) and given clues to solve the murder, along with $10,000. The winner will inherit Westing's estate. People are reminded that it's not what clues they get, it's what they don't have that will really matter. Sydelle makes a shorthand copy of the will, but they aren't allowed to have any others. Turtle thinks the will means they should play the stock market.
Back at Sunset Towers, they're snowed in. People are working on their clues. Everyone's nice to Sydelle until her copy of the will is lost. The judge throws a party for the heirs, where they're all suspicious and spy on one another.
The morning after the party, some heirs meet in the coffee shop to go over their clues, and Sydelle gets her copy of the will back. A bomb goes off. No one's hurt, but it's scary. Over the phone, the judge hires a private investigator, whose voice sounds awfully familiar. Later that day, all the heirs are having dinner in the Chinese restaurant on the fifth floor when a second bomb goes off. This time Sydelle's hurt and taken to the hospital (they're not snowed in anymore). Angela visits her there and wonders about what she's doing with her life, then discovers someone has anonymously given her two more clues.
The characters keep working on their clues: Flora Baumbach and Turtle worry about the stock market, while the judge realizes Mrs. Westing is a big blank spot in her investigation. At Angela's wedding shower, Turtle is reaching over to help her sister with a present when Angela shoves her away. The third bomb goes off. No one's hurt except Angela, who's burned and bleeding. While she's taken to the hospital, everyone else gathers suspiciously in the lobby. When Turtle visits Angela in the hospital, Sydelle figures out that Angela's the bomber. Turtle and Angela talk about inner versus outer beauty: Angela will have a permanent scar.
The judge and Sandy keep doing research, while Theo and Doug spy on Otis. The heirs receive messages to go to another will reading that Saturday night. In the meantime, they have to state their life positions again. Dr. Deere takes Chris for a new medical treatment, Angela gets more clues from an anonymous source, and the judge and Sandy realize that Sydelle is there by mistake. Theo causes a lab explosion, and then discovers that Otis and Crow work at a soup kitchen.
Turtle sets off a fourth bomb in the elevator and she burns her precious braid. She confesses to all the bombings, and is taken to meet with the judge. The judge figures out that Turtle was just protecting Angela, the real bomber. Meanwhile, at the hospital, Angela and Sydelle put all their clues together and figure out that they point to "America."
That Saturday night, the heirs meet for the second part of the will reading. They reveal their answers, and the will says that all of them are wrong. Their teams are broken apart, and the will says they'd better figure out the answer before someone else dies. Sandy suggests they put all their clues together; when they do, they spell out Crow's full name. Suddenly, Sandy dies.
Crow gives herself up to the police as an answer, and everyone else goes back to the judge's apartment. They start to go back over the will and Turtle realizes something's not right, so she calls for a trial. The judge presides, while Turtle interrogates witnesses. Otis says he's a private investigator, Dr. Deere says Westing might not have been dead, and Turtle claims that Crow's not guilty of murdering Sandy. They realize one word is missing from the transcription of the will.
With no missing word, it reads, "The heir who wins the windfall will be the one who finds the… FOURTH" (26.93). Turtle realizes Westing was four people (Westing, Barney Northrup, Sandy…) and she knows who the fourth one is. She also realizes that the true question of the will is who's going to discover Westing's fourth identity. (She keeps all of this secret, though.)
Crow returns, innocent. The heirs read the very last part of the will, which says nothing about money, just that they all get shares in Sunset Towers. The judge says this means they lost the game. Everyone watches as the Westing house explodes in fireworks.
Later that night, Turtle goes to Julian Eastman's house. Eastman is the Westing Paper Products chairman. She addresses him as Sandy and says she won: she found the fourth. Everyone else's lives go on fairly normally. Five years later, everyone seems to be very successful. Angela's gone back to school, Doug's won the Olympics, the Hoos' restaurant is doing well, and Chris, Theo, and Turtle are doing well in school. Several years after that, Turtle stays with Sandy/Westing (as Eastman) while he really dies. She finally wins the prize and goes to play chess with her niece Alice.