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In the short story Dead Man's Mirror, Captain John Lake is the land agent of Sir Gervase Chevenix-Gore.

Some three weeks before the events in the story, he had secretly married to Sir Gervase's adopted daughter Ruth. In order for Ruth to inherit after her adoptive father, John Lake would have to take the Chevenix-Gore name as demanded in Sir Gervase's original will. However, Sir Gervase is about to make a new will which would have disinherited Ruth unless she married Hugo Trent. This gives Lake a plausible motive to murder Sir Gervase before the new will can be signed.


In the ITV's 1993 adaptation of the story, the part of John Lake is played by Richard Lintern. Here he is not a land agent but an architect and Poirot implies that he is the one who designed Hamborough Close, the house of Sir Gervase. Late in the episode when Poirot sees a scale model of "Northgate Development", one of Lake's projects, he notes that the Spanish style design and that of Hamborough Close are the work of the same hand.

Lake is a plausible suspect in the murder of Sir Gervase but for different reasons than in the original story. His motives here are twofold. First, Sir Gervase had called on Poirot to investigate a fraud. In this case it is because he had invested ten thousand pounds into John Lake's mixed development project "Northgate Development" but no progress was being made in the construction. Certainly, Lake appears worried to see Poirot come to Hamborough Close and kept asking if he was on official business.

The second motive is similar to the original story. Here Lake had secretly married Ruth Chevenix-Gore at the beginning of the episode and like in the original story, Sir Gervase was on the verge of making a new will which would have disinherited Ruth unless she married Hugo Trent.