Milo's narrowboat

Milo's narrowboat is both the apparent locked room in the investigation and the physical setting of Sid Towers's death. The puzzle turns on a small, ordinary object attached to the cabin door: a padlock that seems to rule out entry, but can be exchanged to conceal it. Sid's curiosity about that very puzzle brings him to the boatyard at Limpley Stoke, where his encounter with A.J. transforms a clever deception into a murder.

Setting and place in the investigation

Milo's narrowboat is moored at the Limpley Stoke boatyard, where Milo has his houseboat. After a Bloodhounds meeting, Sid drives there; his car is later found at the boatyard. Shirley-Ann Miller reasons that the killer must have known Milo would be away. Milo had gone to the police station to explain the matter of the stamp, and, as she puts it, the Bloodhounds were the only people who knew he would not be home. She attributes that deduction to herself rather than Polly Wycherley, who had not stated it aloud.

That reasoning establishes why the boat matters beyond its role as a crime scene. Its temporary vacancy creates an opportunity, while the small circle aware of Milo's absence appears to narrow the field of suspicion. But Sid's presence complicates the inference: he may have gone there independently, not as the person who planned the crime, but as someone drawn by the mystery. Polly agrees that curiosity could explain his drive, though the evidence does not establish exactly what he intended to do once he arrived.

The locked-cabin puzzle

The boat's cabin seems to present a classic locked-room problem. Milo had explained that the padlock had no spare key; he showed the key and said where he had bought the lock. The apparent security is reinforced by the discovery that Milo's lost keys, recovered from the canal by divers, included the key that fitted his padlock. John Wigfull therefore objects that a second lock cannot explain entry: the lock was on the door when police opened the cabin, and Milo had only one key.

Diamond's answer depends on distinguishing the lock's visible presence from its identity. In his reconstruction, the padlock is hanging outside the door with its shackle open while Milo is aboard. The cabin door is bolted from within, and Milo can use the internal fingerbolts for privacy. A person can lift the open padlock off its staple and replace it with a similar lock, using a different key, without Milo noticing. The important vulnerability is not that the cabin door is left unlocked; it is that the padlock can be changed while it hangs unfastened.

The substitution then works through the ordinary action of leaving the boat. When Milo goes out, he closes the door, places what he believes is his own padlock over the staple, and presses it shut. A padlock of this kind locks automatically, so he does not need to use his key. In fact, the lock now securing the boat belongs to the intruder, who can unlock it and enter. After committing the crime, the intruder can leave and put Milo's original padlock back in place. When Milo returns and uses his familiar key, the cabin appears to have remained secure. The deception leaves the visible evidence of a locked door intact while concealing the change that made entry possible.

Sid's visit and death

Diamond considers that Sid may have gone to the boatyard simply to examine the puzzle for himself. Sid enjoyed locked-room mysteries, and the apparent impossibility of entry would have offered a particular challenge to someone trained in security. Polly accepts that this is plausible. The suggestion remains a reconstruction, but it gives Sid a motive for being at the boat without making him the person responsible for the deception.

In Diamond's account, Sid arrives as A.J. is replacing the padlock. A.J. hears him coming and hides inside the cabin, taking a windlass as a weapon. The plan has nearly succeeded, but Sid's unexpected arrival turns the boat from a controlled setting into an immediate confrontation. Diamond thinks A.J. strikes Sid on the head, probably intending to knock him unconscious rather than kill him; the blow is fatal. The narrowboat's mechanical puzzle thus leads directly to the violence: the lock switch creates the opportunity, and the visitor who comes to understand it interrupts the scheme.

Evidence and significance

The boat's padlock becomes crucial evidence when Diamond reopens the question in front of Wigfull. A padlock found among the bagged contents of the dead man's pockets looks indistinguishable from the one on the narrowboat. Wigfull recalls his earlier substitute-lock theory, which he believed Diamond had disproved, and points to the boat's original lock and its recovered key as reasons the case seemed closed. Diamond insists on testing the physical evidence, bringing out Milo's lock and its key and asking for the bag containing Rupert Darby's keys. The scene shifts the investigation from an abstract impossibility to a comparison of particular locks and keys.

The boat's importance is therefore double. As an object in the investigation, it supplies the door, staple, internal bolts, and padlock whose arrangement makes the apparent locked room possible. As a place in the plot, it brings together Milo's absence, Sid's independent curiosity, and A.J.'s effort to conceal his actions. Its security is not simply disproved; it is shown to have been staged. The lock that reassures Milo and frustrates the investigators is also the instrument of the deception, and Sid's attempt to solve that deception is what exposes him to the fatal encounter.