Turtle Wexler, going into the old Westing house on a dare on Hallowe'en, finds what looks like a dead body. She assumes that it is Sam Westing, and the evidence given by the doctor in (the real) Sam Westing's employ bears this out. The doctor is, of course, lying, and has been providing medical "evidence" for Sam Westing for years in order to help him play through his elaborate deception games.
This "discovery" and "evidence" deceives everyone, especially the "Eight Pairs of Heirs" (the sixteen people named in Sam Westing's will who must compete for his fortune in a strange and sometimes dangerous game of the title) that Sam Westing is dead and his fortune is up for grabs. The "body" was really just a wax effigy, not an actual dead person.
Turtle not only finds the "body", but in the end she is the only one who ends up knowing the real Sam Westing after he "died" (as the doorman) the second time (and became Julien Eastman). Turtle, who ends up being the main character of the story, eventually comes to know all the secrets of Sam Westing, even though she was the first one actually deceived by him.