James D'Arcy as Derek Kettering in Agatha Christie's Poirot
In the novel The Mystery of the Blue Train, The Honourable Derek Kettering is the son of Lord Leconbury. He is married to Ruth and the son-in-law of Rufus Van Aldin.
Role in the novel (warning, it contains spoilers)[]
He is the lover of Mirelle. He was arrested for the murder of Ruth. If Ruth had died later, she would have divorced from Derek. However, Derek wasn't responsible for the death of Ruth. The real killer of the case was Ada Mason, Ruth's maid.