In Costume Party Detective, you have been invited to a costume party. Because of the elaborate costumes, you do not know anyone's identity except your own. Roam the party, mingle, have some punch, and try to identify all of the guests. Be the last guest partying, and you win!

Players are secretly assigned one of twenty unique meeple characters at the beginning of the game using character cards. On your turn, you roll the die and move any character on the board according to simple movement rules. If you roll black, however, you must act! You may either remove one meeple from the room your secret character is in (thus revealing vital information about your identity), or you may draw the top card off the character deck. The character on the card "leaves the party" and is removed from the game. You may draw a character card at most three times, after which you must remove a character in the same room as your secret character. If you are the only character in a room and you have already drawn three character cards, you are dishonored and must unmask yourself!

If your character is removed, you reveal your character card and are eliminated from the game. The last player to have a character in the game wins!