The Double Clue - S3.E7

Japp (who is really worried that failure could cost him his job) seeks Poirot's help in investigating a series of jewel thefts, and two remarkable things happen. Poirot falls in love (with the beautiful and clever Countess Vera Rossakoff) and he also meets a criminal intellect which he finds worthy of him. As the countess is his main suspect, Poirot spends most of his time with her - which for once leaves Hastings and Miss Lemon to do most of the actual detective work.

In the end, this case may have to be put down as the one that got away.