Sid Towers is killed after arriving at Milo Motion's narrowboat while Gilbert Jones is replacing its padlock. The death is the unintended, fatal turn in Jones's elaborate locked-room deception: Sid's curiosity brings him to the boat at the decisive moment, and Jones's attempt to get away ends in a blow that kills him.

The padlock deception

Jones's scheme depends on an ordinary feature of a padlock: once its shackle is pressed shut, it locks automatically, without a key. While Milo is aboard his boat, Jones approaches and substitutes his own padlock for Milo's, leaving Milo unaware that the lock has changed. Jones then waits for an opportunity to enter.

When Milo later goes out, he closes the door and secures what he believes is his own padlock. He does not need to use a key; he simply places the lock over the staple and presses it closed. But the lock now attached to the boat belongs to Jones, who can open it. Jones can therefore enter the cabin, commit the crime, and leave, then put Milo's original padlock back on the door and close it. When Milo returns and unlocks the door as usual, the cabin appears to have remained secure. The switch makes access possible without leaving an obvious sign that anyone has entered.

Sid arrives at the boat

The carefully arranged mechanism fails when Sid, fascinated by the apparent locked-room puzzle, drives out to inspect the narrowboat. He is particularly interested in such puzzles and is described as a Dickson Carr enthusiast. His independent curiosity takes him along the towpath at the moment Jones is replacing the original padlock. Sid is not the intended victim of the scheme; his arrival interrupts Jones in the act of concealing the lock switch.

In Peter Diamond's reconstruction, Jones hears Sid approaching and hides inside the cabin. He takes the boat's windlass as a weapon. The sequence links the deception directly to the killing: the padlock switch has made the cabin accessible to Jones, but Sid's unexpected arrival turns that concealed access into an immediate danger for its author. The account does not establish that Sid saw Jones before the attack; Jones later says that Sid had not seen him.

The fatal blow

Jones strikes Sid from behind. He later admits that he hit him, but says he meant only to make his getaway, not to kill him. Diamond likewise reconstructs the blow as an attempt to knock Sid unconscious, while emphasizing that a blow to the skull can have a fatal result. The death is therefore presented as unplanned, though it follows from Jones's decision to use violence when the padlock operation is interrupted.

That distinction separates the original scheme from its consequence. Jones had planned the theft, the padlock substitution, and the opportunity to enter the boat; Sid's death was not part of that plan. Jones is described as angry and frightened when Sid appears, and he panics. A crime that had seemed close to succeeding as a calculated deception becomes a killing followed by efforts to hide what happened.

Misinterpretation and cover-up

After Sid is killed, Jessica suspects A.J. is responsible and does not want the police to discover it. She hopes that he may escape detection if others conclude that Sid stole the stamp and staged the locked-room trick. That explanation has apparent force because Sid is known for his interest in locked-room puzzles; his expertise can be turned into a misleading reason to suspect him.

The mistaken theory also widens the imagined opportunity for the attack. Rather than pointing to someone who knew about the padlock switch, it permits the possibility that anyone on the towpath could have attacked Sid. The writing on the paper bag seems to support the claim that Sid composed the riddles, reinforcing the impression that he was behind the puzzle and its solution. In the retrospective account, these details explain how suspicion could be diverted from Jones after the killing; they do not change the fact that Sid encountered Jones during the lock replacement.

Significance in the investigation

When Peter Diamond lays out the mechanics, the group follows how the two padlocks enabled Jones to enter and leave without Milo understanding how the cabin had been accessed. The explanation resolves the apparent contradiction at the center of the boat puzzle: the door could appear secured with Milo's lock while Jones had used his own lock to gain entry. The murder is thus bound to the physical detail of the substituted padlock, rather than to an inexplicable means of entering a closed cabin.

Jones eventually concedes that Diamond has worked out the plan, while maintaining that he had not intended to kill Sid. That admission confirms both the deception and the accidental character Jones claims for the fatal blow. Sid's curiosity does not make him the perpetrator, as the misleading theory suggests; instead, it brings him to the scene at precisely the moment that exposes Jones's operation. His death is the consequence that turns a clever locked-room fraud into a homicide and makes concealment necessary.