How the Light Gets In by Louise Penny is the ninth book in the Chief Inspector Armand Gamache series. The main characters in the series are Chief Inspector Armand Gamache and his sidekick, Jean-Guy Beauvoir. Gamache and Beauvoir have worked closely together for almost 15 years. However, at the beginning of this story, a rift now separates them. Jean-Guy is working under Chief Superintendent Francoeur, Gamache's number one rival. This makes for an interesting twist later in the book.

The author has interwoven three story threads. The main plot centers around Gamache as he searches for the person who leaked a video of a deadly raid. During the search, he accidentally stumbles upon a plot to destroy the peace of Quebec. Along with saving lives and avoiding a tragedy in his Canadian providence, other subplots evolve as Gamache solves the mystery of the murder of the last of the Ouellet Quints and uncovers the secret the girls had been hiding for years. The book concludes with Gamache and his wife celebrating the second wedding of their daughter, Annie Gamache, to Jean-Guy Beauvoir.