A man involved in a car crash is found to have fifty thousand pounds worth of diamonds in his stomach. Steed investigates the man's joke shop, Big Laugh, and then the offices of multi-millionaire Litoff. The burial of one of Litoff's Great Danes is not quite what it seems.

Thanks to the clever briefing of the Lakin brothers a man charged with treason is found not guilty. Mrs Gale believes that Steed is implicated, and contacts the newspapers. Steed threatens her: either withdraw her allegations or pay the consequences. Soon Steed is on trial for Gale's murder.

When Lucas, an agent investigating something subversive, gets off his train at the wrong station he is murdered. Has the crime got anything to do with a mole at the admiralty, a train ticket and a fiendish plot to kill the Prime Minister?