The Dungeon House by Martin Edwards is the seventh (and final) book of the Lake District mystery series set in the contemporary Lake District of England. Hannah Scarlett is in charge of the Cold Cases detectives in the Cumbria police. Her lover Daniel, a retired academic, is the son of Hannah's former mentor, the late detective Ben Kind. Les, one of Hannah's experienced detectives, cautions anyone investigating a case to "understand the relationships in order to solve the crime". Indeed, each book of the series details an intricate web of relationships leading to crime.
Twenty years ago Malcolm Whitely lived at Dungeon House, a luxurious mansion in Cumbria, with his stunning wife Lysette and teenage daughter Amber. Malcolm made a fortune by fair and foul means, sold off his business at huge profit, and dedicated himself to a life of drunken luxury. The shady characters who bought his business are threatening him, but he just stays drunk He's unreasonably possessive and jealous of anyone who pays attention to his wife. He's certain she's having an affair with her private art teacher, Scott Durham. At Malcolm's annual house party, he's in a drunken stupor. After the party, when his wife attempts to leave him, he shoots her. Next day, police find Amber has fallen to her death in the quarry, and Malcolm shot himself.
Three years ago, Lily Elstone disappeared. Her father Gray Elstone was Malcolm's accountant.
In the present day, Elstone has Nigel Whitely as a client. Nigel is Malcolm's nephew. He lives in Dungeon House, renamed Ravenglass. Nigel's daughter Shona has disappeared. He appears on television appealing to all to report any sightings, and urging her to return home. Joanna Footit's crush on Nigel is rekindled when she sees him on TV. She and Amber Whitely had been friends. Joanna and Nigel, Robbie Dean and Carrie North were in a car crash not long after the Whitely shootings. Carrie didn't survive the crash, Robbie was disabled. Joanna had a mental breakdown following the crash, and moved away from Cumbria. But now she regains self-confidence, and goes back for a holiday. She hopes to finally establish the relationship she always wanted with Nigel. She stays at a guesthouse run by Carrie North's father, decorated with paintings by Scott Durham.
When Joanna meets up again with those she knew twenty years ago, she stirs memories long hidden. Hannah's cold case is Lily Elstone's disappearance, but detective Les sees many ties between the Whitely deaths, Lily's disappearance, and Shona's. Soon Hannah and Les are investigating them all. A particularly unpleasant task for Hannah is interviewing Cheryl, Lysette's best friend (Ben Kind had left his wife and children to live with Cheryl). Hannah learns key information about the past from Joanna, later gets help just-in-time from Daniel Kind.