Challenge was the tenth and final Bulldog Drummond novel. 

When Colonel Henry Talbot summons Bulldog Drummond and Ronald Standish, it is to inform them of the mysterious death of one of their colleagues, a millionaire. There was no sign of any wound, no trace of any weapon when they found him. What strikes Drummond and Standish is why a millionaire would have been travelling on the same boat - arguably the most uncomfortable crossing he could choose and very out-of-character.