Plot summary
The Barnabas family is no stranger to mystery, one of the founder's nephews, Tom Barnabas, having disappeared from the street in broad daylight never to be seen again. When it is remarked, at a Sunday evening gathering held by Gina Brande in her flat next door to the offices, that Paul Brande, her inattentive husband, has not been home for three days, no-one finds it too remarkable. Shortly after the arrival of his old friend Albert Campion to look into the vanishing, Paul's younger cousin Mike, in love with Gina, goes to the vault to fetch some papers for the eldest cousin, the Barnabas Managing Director John Widdowson, and returns looking shaken, but saying nothing.
Next morning, Paul's body is found sprawled in plain view in front of the vault. The doctor who is called immediately recognizes that Paul has been dead for several days, and a decision is made to move the body from where it was found, destroying clues. Mike is sent to inform Mrs Brande and seen comforting her tenderly. Campion investigates the scene of the crime and finds a recently broken ventilator to the rear of the vault, which leads to a next-door garage. Questioning staff, he discovers that the position of the body made it impossible for Mike to have missed him the night before. The police find a length of rubber pipe stained with soot.
At the inquest, the doctors and police reveal that Paul was killed by carbon monoxide inhalation, the pipe having been used to connect the exhaust of Mike's car to the vault ventilator. A neighbour testifies she heard the car running in the garage from six to nine on the night Paul disappeared. Mike says he was out walking the streets until eight, and that around nine had started the car to warm it up by running the engine for a few minutes before going for a drive. Gina's housekeeper speculates too freely about her mistress's relationship with Mike, and at the end of the inquest Mike is arrested for the murder.
Campion befriends Ritchie, an odd and rather awkward cousin - the brother of the vanished Tom, and with his help questions Miss Netley, Paul's suspicious secretary. He tracks down Paul's mistress, but finds she knows nothing, except that he missed an appointment with her on the night he died. He learns of a valuable unpublished manuscript of a play by William Congreve owned by the firm and regarded as a financial asset, which Paul had hoped to display, and of a visit Paul paid to a London district on business concerning a key. From his manservant Lugg, he learns that a renowned underground key copier lives in the area mentioned, and together they investigate, Lugg's criminal past helping to persuade the man to help by providing a copy of the key he made for Paul.
Campion visits the Barnabas offices with Ritchie's help and finds someone in the vault. They fight, and Campion subdues the man, who turns out to be the accountant, Rigget. Rigget confesses to having made a copy of the vault key and sneaking in at night to look for any valuable information. On the night after Paul's death, he had also entered the room and found Paul curled up and dead in a corner, and the vault unlocked; he locked it using Paul's key. He also took the vault key from inside the door, locked it on the outside when he left, returning the key to its normal place.
The trial begins the next day, and Campion, exhausted from his long night, attends. Things look bleak for Mike, but Campion has found out from an uncle working at the British Museum that the document in the vault is not the original manuscript. He visits John Widdowson's flat with Ritchie, looks around, questions the maid, and leaves a note for Widdowson enclosing a page from the facsimile manuscript. Widdowson calls him later, agrees that the manuscript he found was a facsimile, but that the original is safe in one of the firm's other offices, telling him to confirm the fact by opening a certain locked cupboard. Campion goes there, still weary from a lack of sleep, and almost hurls himself at the jammed door to force it open. Becoming suspicious, he kicks it instead, and discovers it leads outside to a terrible drop. He realizes that he too was to be killed by Widdowson for the same reason he killed Brande, to prevent exposure that the play, which is the company's financial asset, is not an original.
During the court proceedings against Mike on the next day, the case is suddenly called off. Widdowson has been found in his bath, an apparent suicide caused by carbon monoxide fumes from a gas water-heater. Campion learns that the windows had been were wedged shut from the outside, that the heater had been manipulated, and that Ritchie has vanished. Later, holidaying in France with Mike, Campion finds the long lost Tom Barnabas, alias Pierre Robert, who tells him that instead of his legacy he had taken the original manuscript to buy a circus, in which Campion sees Ritchie performing, now finally in his element as a clown. Campion realizes that Ritchie's quietly avenging the murder of Paul by killing Widdowson gave him the role of "the king's executioner."
