The novel is notable for the fact that Poirot never visits any of the crime scenes or speaks to any of the witnesses or suspects. He is challenged to prove his oft-made boast that a crime can be solved by the exercise of the intellect alone. The novel also marks the return of partial first-person narrative, a technique that Christie had largely abandoned earlier in the Poirot sequence but which she had employed in the previous Ariadne Oliver novel, The Pale Horse (1961). There are two interwoven plots: the mystery Poirot works on from his armchair while the police work on the spot, and a Cold War spy story told in the first person narrative.