Another devilishly complex mystery for the great Hercule Poirot... Hercule Poirot travels by luxury coach from London to Alcyone Hill, a rural, private settlement. He has been invited by Richard Devonport to help prove that his fiancée Helen is innocent of the murder of his brother Frank. 

However, his request comes with a strange condition: Poirot must keep the real reason for his presence there a secret from the rest of the Devonport family. On the coach, a visibly upset woman desperately asks to disembark, insisting that she will be murdered if she remains in her seat.

She changes seats and the rest of the journey continues without further incident. However, Poirot has a bad feeling and his fears are soon confirmed; another body is discovered at the Devonport house, with a macabre note that reads: You sat in a seat you should never have sat in...
Could this new murder, as well as the strange incident on the coach, be clues that will lead to solving Frank Devonport's murder? Will Poirot be able to find the real killer in time to save an innocent woman from the gallows?