The outing seems innocuous enough. Two cousins, Derek and Nigel Burtell, have agreed to a canoeing voyage up the Thames, the object being to restore Derek's failing health. As it happens, Derek is shortly to inherit £50,000 from his grand father. In view of the young man's deteriorating physical condition, however, the Indescribable Insurance Company (Knox's nod to the "Lord, what fun!" school of detective fiction) has insured his life, in the event he does not reach his 25th birthday, the scheduled date of his windfall.

The journey upstream goes without a hitch: however, the return trip is destined to be less than idyllic. Nigel leaves the canoe at Shipcote Lock to take an exam at Oxford. Before he can rejoin his brother, the canoe is discovered adrift below the lock, empty and with a jagged gash in the bottom. Derek is nowhere to be found.

Suspecting foul play, the Indescribable sends Miles Bredon, the firm's premier investigator, to look into the case. Can the pastoral reaches of the upper Thames be harboring a murderer? Has Derek met with an accident? Will the company have to pay up? To complicate matters, Nigel Burtell has also vanished.

Thus begins an urbane and literate mystery, delightfully tongue-in-cheek in tone, that offers mystery buffs ample opportunity to match wits with the imperturbable Bredon and to puzzle over a cluster of clues--a set of strange photographs, a baffling cipher, a trail of naked footprints in the mud near the lock. The author's poetic evocation of the river and the countryside through which it flows provides a peaceful counterpoint to the dubious deeds under investigation.