The Revenge of Neptune King is the fourth book in the series. Here we meet Shark’s older brother, Lancelot, who has come to England to take up a position on the London Times. He soon proves his usefulness by sending them new clients, which is fortuitous as Neptune is now broke, having paid his entire fortune to Shadow as ransom for his sister and Shark in The Adventures of Neptune King. Their first case, The Flower People sees them initiated into a cult run by Vincent Flower who uses hypnotism to control followers for unpaid labour and encourage them to sign over their assets, while abusing the women. Neptune falls victim to the hypnotist, while Shark is imprisoned, leaving Rachel to find a way to rescue them on her own. In One down, Two to Go, Neptune, unable to afford petrol for his car, borrows twenty pounds from Shark only to see his own initials on the note. NK in the right hand corner. Every note in the ransom he paid to the Shadow had been similarly marked. Excited by this new lead, they follow the trail of the money to Epsom racecourse and with the cooperation of the track’s manager they eventually get a lead on the punter that paid out the marked money. This guides them to the hit and run driver who critically injured Neptune’s brother-in-law, and eventually to the Shadow himself as they follow a trail that sends them to Paris, where Shark’s dalliance with a couple of French ladies of easy virtue gives them a clue to the Shadow’s true identity. The case comes to a head at the Shadow’s 12th century chateau in Quercy, Bordeaux, where they risk life and limb to capture him in a climatic showdown of good versus evil.