Have you ever seen the movie Clue or read an Agatha Christie novel? The party will bear a striking resemblance. There are different characters and personalities like a famous movie star, a sports figure and a crooked politician. The party will follow a loosely designed plot; perhaps it is a 1950's sock-hop or maybe some big rich snob is throwing a party in the 1940's. The guests show up in costume and "act" out their parts . . . only they have no lines to memorize; they take whatever course of action they think their character might take.
Just think of the classic mystery movies you've seen and/or the mystery novels you've read. You are introduced to the characters, there is some premise for the characters being brought together, a crime is committed, there is an attempt to gather information or clues, and finally there is usually an explanation of how the crime was committed (a solution is given). These same steps occur at the party: