In the early days of World War II, a man wakes in a country hospital to find he cannot remember anything prior to his arrival except that he has something vital to do, somehow connected to the number fifteen.

After seeing Anscombe home, the two continue to the house of Lee Aubrey, the head of a local scientific research body called the Institute, with whom they are staying.

Campion, the last person to see Anscombe alive, accompanies Hutch to the scene of the death, and the two are joined by Pyne, a fellow guest of Aubrey's.

Later that night, Hutch arrives at Aubrey's home in secret to meet with Campion; the two had previously arranged a covert mission into the nearby town of Bridge.

Hutch smuggles Campion into the Council Chamber, a meeting place for a local organisation of dignitaries known as the Masters of Bridge, which is built into caves in a hill overlooking the town.

Bluffing his way through an investigation of the site, Campion finds an agenda for a meeting which mentions Minute Fifteen, and Anscombe's sudden retirement from the order.

The hotel is surrounded by both police and criminals, so Campion escapes over the roofs and catches a train to London where he meets Sir Henry Bull, a high-ranking member of HM Treasury and one of the Masters of Bridge.

When Amanda arrives to post bail, Campion finally realises what the scheme he must foil is; an attempt to flood the United Kingdom with large sums of counterfeit money, which would devalue the pound sterling, cause inflation to skyrocket and destabilise the economy and the government.

The money is to be posted at the same time as news about the Minute Fifteen war loan, disguised as a social security payment to the poor.

Operation Bernhard was a real life German plot to flood wartime Britain with counterfeit currency – however this did not become public until after the end of the war, so Allingham was unaware of it at the time the novel was written.