However, the attempt goes wrong when he falls not into the water but onto a passing barge, breaking his legs and killing a dog which was on the deck.
Some years ago, Purvis was approached by a Soviet spy named Rashnikov, who asked him to work as a double agent.
However, Purvis also recalls that Rashnikov had told him to tell his British masters that he was being recruited, effectively setting up a double bluff ahead of time.
The next scene opens at Purvis's memorial service – he has succeeded in committing suicide by rolling his wheelchair off a cliff at the rest home.
Blair ponders: "One asks oneself, with the benefit of hindsight, was Clifftops the ideal place to send someone with a tendency to fling themselves from a great height to a watery grave?
Of course at the time one didn't realise it was a tendency..." In the closing scene, the whole structure is explained by the agency's unnamed chief, although even this explanation remains dizzyingly complex.
Despite being perhaps the central character of the play – certainly he has the most lines – he remains a somewhat ineffectual figure, happy to allow events to follow their course.
Hogbin is the polar opposite of Blair – doggedly determined, prone to panic and seeing conspiracies at every turn.
She runs a donkey sanctuary, occasionally appropriating her husband's study as an operating theatre for her injured charges.
The Dog It Was That Died has been described as Stoppard's 'le Carrécture', and it takes much of its mannered approach from John le Carré's work.
The characters of Blair and Purvis are contrasted skilfully – one the benignly complacent bureaucrat, the other a deeply principled fighter for his beliefs.
Virtually all the characters in it have a pronounced eccentricity of some kind: Blair's clocks and his folly; his wife's donkey sanctuary and casual affair with her husband's superior; the chief's regular smoking of opium; the obsession with rare cheese of the vicar who carries out Purvis's memorial; and Seddon's fascination with guano.
The words also occur in a speech by Wormold, the lead character in Graham Greene's 1958 novel Our Man in Havana – also an entertaining tale of Cold War espionage.