On the Orient Express, there were thirteen passengers. One of them was a millionaire named Samuel Edward Ratchett. He was killed in his locked compartment by one of the remaining twelve passengers.
After discovering Samuel's body, Hercule questions everyone who's on the train. He has M. Bouc gather people one by one into the restaurant car for questioning. Everyone has their own story about what they were doing that night.
M. Bouc and Hercule start to put together the summaries of each person's testimony. More evidence comes up later on when Mrs. Hubbard finds a bloody knife in her sponge bag. Hercule and M. Bouc also investigate luggage for a silk kimono.
Hercule finds out that everyone was involved in the murder. Mrs. Hubbard, revealed as Linda Arden, tells Hercule that his first theory about a stranger entering the stopped train and killing Samuel as false. She says that they all were in on the murder.
Hercule talks with M. Bouc and Dr. Constantine to come up with something to tell the police.
They agree to tell the police that a stanger entered the train while it was stopped and stabbed Samuel to death. Hercule chooses to protect the family.