In Greek mythology Tantalus is a son of Zeus and a mortal woman who offends the gods by killing his own son and serving him in a dish to the gods. The gods restore his son to life and Tantalus is punished by being forced to stand in a pool of water that he is never able to drink from, no matter how thirsty he is.
Sullivan says that he and the other undercover officers are all using the alias "Inspector Trueman" to hide their real identities. A character with the same name, played by Adrian Scarborough, appears as Sullivan's rival in The Owl of Minerva (2015), his final appearance as a regular character.