As a criminalist you try to solve a murder by interrogating a witness. You can ask the witness about the characteristics and the motives of the culprit as well as about the found evidence.
A game consists of several rounds. In each round one of the players is the witness and randomly selects one of the cards indicating the guilty person. The criminalist whose turn it is shows the witness one of the characteristics, motives or evidence cards. The witness then answers (mostly truthfully, though there are exceptions, when he is allowed to lie or refuse the answer). Based on these answers the other players try to find the culprit. The aim is to be the first person to guess correctly. Then the next player becomes the new witness.
The game ends after all players have been the witness the same number of times.