Set in the 1920s, Post Mortem puts the player in the role of MacPherson, a retired private detective, whom is hired by Sophia Blake. His assignment is to find out who killed her sister and brother-in law.
The gameplay is similar to games like AmerZone: The Explorer's Legacy and other first-person adventure games, where the player goes hunting around for clues and interacts with other characters. During cinematics, the game moves the perspective to the third-person and offers different dialogue choices. The storyline is affected according to chosen lines.