The following version of the novel was used to create this study guide: Penny, Louise. All the Devils are Here. Minotaur Books, September 1, 2020. Kindle.

In the novel All the Devils are Here, the last installment in Louise Penny’s Chief Inspector Gamache series, Armand Gamache confronts the devil within himself as his family is threatened by greedy, powerful professionals trying to cover up a fatal engineering flaw. Armand, and his wife, Reine-Marie, were in Paris awaiting the birth of their new granddaughter when Armand’s godfather, Stephen Horowitz, was critically injured when he was hit by a delivery truck. Armand, who was with Stephen at the time, knew Stephen was hit intentionally. Armand’s loyalty to his godfather made him determined to find out who had hurt Stephen and why.

The day after Stephen was hurt, Armand and Reine-Marie discovered that he had been staying in a suite in Hotel George V instead of his apartment in Paris. It appeared another man had been staying with Stephen in that suite. Armand and Reine-Marie next visit Stephen’s apartment to determine why he was not staying there. Inside they found the apartment had been ransacked and the body of Alexander Plessner, a man who Armand does not recognize, laying behind a couch. Armand and Reine-Marie both smelled a hint of cologne in the air, proof that the killer might still be there. Armand tried to trap the killer in the apartment, but he escaped through a second door in the kitchen. Armand and Reine-Marie both realized later when the met with Claude Dussault, the head of the police in Paris and Armand’s friend, that he wore the same cologne.

When looking through a box of Stephen’s personal items given to Armand at the hospital, he, Dussault, and Armand’s son-in-law, Jean-Guy Beauvoir discovered an agenda for the board meeting of GHS, an engineering firm for which Jean-Guy worked. They wondered if perhaps Stephen had found some evidence of an engineering flaw in items made by that company. Jean-Guy went to his office to see what he could learn by logging into a co-worker’s computer. Their suspicions seemed to be confirmed when Jean-Guy was confronted by a guard questioning why he was working on the weekend. While Jean-Guy was talking to this guard, he noticed files being erased from his co-worker’s computer.

The continued investigation determined that GHS was mining neodymium, a rare earth mineral, from a mine in Patagonia. Neodymium could be used to make strong magnets, but had several faults also. It broke down if it got too hot or too cold. It could also fracture under pressure. Armand and Jean-Guy deduced that Plessner, an engineer, had approached Stephen about problems with GHS products. They found a list of dates in Stephen’s calendar that corresponded with seemingly unconnected deadly accidents.

Stephen liquidated all his possessions, including selling his homes and his collection of artwork, to purchase a controlling interest in a tool and die company and a smelter, both owned by GHS. Stephen additionally approached one of the board members, Alain Pinot, to buy his seat on GHS’s board of directors. Armand determined that Pinot was the one who betrayed Stephen and raised the alarm with GHS security.

The novel climaxes with Armand’s son, Daniel, kidnapped and held hostage in Stephen’s apartment by the same people who tried to kill Stephen and did kill Plessner. Armand was ordered to bring back the evidence that Stephen had against GHS before the GHS board meeting or his son would be killed. With the help of his friends and a guard who has changed loyalties, Armand is able to find the evidence and save his son while at the same time making public the crimes committed by GHS.