Frank Jarvis is an ambitious man who marries a rich widow and later plans her demise. He locates his identical twin Mark in Canada and persuades him to visit him in the UK. His intention is to use his brother to set up an alibi. Mark is completely unaware of the plot and nor does he know that Frank has married. So in all innocence Mark visits England and gets drawn into a cock and bull story given him by Frank. Frank then murders his wife Isobel and goes to the designated meeting place where he has asked Mark to meet him in his car. However spotting a police car in the vicinity, he shouts to Mark to hurry, and since the latter is used to driving on the right, they are involved in a fatal accident where Mark gets killed. Frank's alibi has now been blown sky high. When questioned at the scene by the Police, he suddenly has what he considers to be a brainwave. He states that he is MARK Jarvis! He is accepted by everybody as Mark and feels confident that he will eventually inherit the considerable fortune left by Isobel. As Mark he would be next of kin to Frank, as his wife had no children from her first marriage. However Frank is unaware of a crucial difference between he and his brother, and an astute detective is soon aware. But Frank then disappears seemingly without trace.