The story starts in 1911 with the mysterious disappearance of one of the members of the Barnabas family, owners of a leading publishing house in London.

After the initial investigation, the mystery soon disappears from the public's mind, and the remaining cousins continue the family business.

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.

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.

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.

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.

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.

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 realizes that Ritchie's quietly avenging the murder of Paul by killing Widdowson gave him the role of "the king's executioner."