The Beresfords' old friend Mr Carter, from a government intelligence agency, arrives bearing a proposition for the adventurous duo. They are to take over 'the International Detective Agency', a recently cleaned-out spy stronghold, and pose as the owners so as to intercept any enemy messages coming through. In the meantime Tommy and Tuppence can take on cases as this detective agency, an opportunity that delights the young couple. They employ Albert, a young man also introduced in The Secret Adversary, as their assistant at the agency.
The two tackle a series of cases – mimicking in each the style of a famous fictional detective of the period, including Sherlock Holmes and Christie's own Hercule Poirot.
