The locked-room condition aboard the Mrs. Hudson

The locked-room condition aboard the Mrs. Hudson turns the discovery of Sid Towers’s body into a problem of evidence as much as entry: the cabin appears sealed, the stern padlock seems resistant to interference, and Milo Motion says the only key was with him. Peter Diamond repeatedly tests the logic of this arrangement, but each apparent solution depends on an assumption that later proves unsafe. The locked room is therefore not simply a physical enclosure; it is the investigation’s unstable account of what happened, and of whom it is reasonable to trust.

The cabin and its apparent security

Diamond’s working reconstruction rests on five connected claims. Milo Motion locked the boat when he left; the key stayed on his person; his keys fitted that padlock and no other, with no second key available; the cabin’s only other entrance, at the fore end, was bolted from inside; and the stern padlock remained in place when Milo returned with John Wigfull and opened it to discover Sid Towers dead. Taken together, these details leave no obvious route for a killer to enter and depart. The body is inside, but the doors appear to rule out the ordinary passage that would explain its presence.

The lock itself seems to strengthen that contradiction. It is described as a German-made, high-security close-shackle padlock, with more than six million possible key variations; its manufacturer claims each lock sold in Britain has a unique pattern. The forensic laboratory reports no flaw in the mechanism and no sign of tampering. That examination lends the hardware an almost absolute authority: the lock appears to have done exactly what it was designed to do. Diamond, however, cannot treat a sound mechanism as proof that every human statement about its use is sound. His reasoning repeatedly returns to the qualification that the five propositions hold only if Milo has told the truth.

The key, the testimony, and the second entry

Wigfull presents the lock to Arnold Musgrave with the energy of a demonstration. The padlocks were supplied with two keys, he explains, but Milo said he had accidentally dropped one into the canal. The remaining key was carried on the key ring in Milo’s pocket with his car keys; Wigfull produces it, inserts it, and opens the lock. He adds that he personally watched Milo unlock the boat on Monday night before discovering the body. In Wigfull’s account, the key’s practical uniqueness and Milo’s possession of it make the dead man inside the cabin an impossible crime.

That presentation establishes what Wigfull believes he witnessed, not an independent explanation of how the murder occurred. Arnold Musgrave tries the mechanism himself, then raises the possibility that Milo might have left the boat earlier without securing the padlock. The question exposes a gap between the lock’s reliable operation and the history of its use: the hardware can be tested in the room, but the earlier circumstances still depend on testimony. Later, Milo insists that someone must have entered twice, once to place the stolen stamp inside his copy of John Dickson Carr and again in connection with the murder. Diamond rejects the claim that there is evidence of a break-in. Neither door nor window shows interference, and nothing is broken. The stamp’s appearance and the murder thus intensify the locked-room problem without proving that the cabin was forcibly entered.

Competing explanations and the return to the puzzle

Julie Hargreaves argues that the stamp thief and the killer must be the same person. Both, in her view, managed to get into the boat without disturbing the lock; she rejects the idea that two independent intruders separately solved the same difficult problem. Her proposal gives the two crimes a common mechanism and redirects the inquiry from abstract speculation toward face-to-face questioning of the suspects. It remains an argument about the likely identity and method of the intruder, however, rather than a demonstrated account of entry.

A theory that Sid Towers had fitted a different padlock offers a more concrete way to dissolve the contradiction: if the lock at the boat were not the one associated with Milo’s keys, the key evidence might no longer mean what the investigators thought it meant. But when the keys are identified as Milo’s original keys, Diamond concludes that there was no substitution and that the padlock is the original one. Wigfull’s proposed explanation of the locked room consequently fails. The reversal does not reveal a route into the cabin; it removes a theory that had seemed to provide one.

An unresolved condition

By the time Arnold Musgrave, Wigfull, and Diamond meet again, the neat and plausible account they had assembled has been unravelled. Musgrave’s summary returns them to the locked-room mystery with which they began, while Wigfull’s response leaves open the possibility that the appearance of failure may itself be misleading. The condition remains unresolved in this sequence: the original padlock, the apparently bolted second entrance, the unbroken doors and windows, and the body inside continue to resist a single explanation.

The puzzle’s force comes from the conflict between physical evidence and human assurances. The laboratory can report that the lock is sound; a key can open it; Wigfull can describe watching Milo use it; and Milo can maintain that the key never left him. None of those facts, separately, supplies a complete account of the murder. Each proposed solution changes the interpretation of the same closed cabin, then falters when another detail is checked. The locked-room condition therefore keeps the inquiry suspended between an apparently certain mechanism and a history of access that its evidence has not yet settled.