Almost as soon as they arrive tired and weary at Waterloo after a fruitless quest in Southern Europe, Mark and Mike Errol are accosted by two men who claim that a certain Mr Loftus wishes to see them in the booking-hall. At first the Errols deny all knowledge of the man, but as agents of the celebrated Department Z, they realise there is no point in maintaining their denial. As the Department's top agent, the name of Loftus is virtually a byword.

The four retire to the booking-hall, but what takes place there makes the Errols very glad of the sudden appearance of Spats Thornton, a fellow agent. It is within the next few minutes, however, and in Thornton's presence, that a murder is committed...