From the introduction:
You...are passengers on the luxurious Oriental Express. Your fellow passengers number several unsavory characters, almost certainly up to no good. No one knows who they can trust, as everyone seems to have an ulterior motive.
Unexpectedly, the train is snowed in and one of the passengers is murdered. Fortunately, the famed detective Pardot is on board. With the help of [the characters], the identity of the murderer should be discovered by the time the police arrive.