Summer, 1915. As Zeppelins rain death upon the rooftops of London, eminent members of society begin to behave erratically: a Member of Parliament throws himself naked into the Thames after giving a pro-German speech to the House; a senior military advisor suggests surrender before feeding himself to a tiger at London Zoo; a famed suffragette suddenly renounces the women's liberation movement and throws herself under a train.

In desperation, an aged Mycroft Holmes sends to Sussex for the help of his brother, Sherlock.