Stephanie Zimbalist stars as an American tourist in Cairo (with Spain standing in for Egypt). She runs across a mysterious murder, and an odd encounter with a stranger; all this occurs even before she sets foot on a luxury liner with an all-suspect passenger manifest. It is established that the murderer, who is after a cache of valuable diamonds, is an internationally notorious criminal--but which one of the passengers is he (or she)? Maybe the villain romantic lead Simon Dutton, the 'man in the brown suit' of the title...but don't discount such special guest suspects as Rue McClanahan and Tony Randall.