Once again, the distinguished mathematician Henry Rathbone is approached to help solve a mystery. This time, the call comes from an old friend whose son has gone missing--and just before the Christmas holiday. Now in his early twenties, Lucien has known trouble for some time and has often fallen victim to the vices of drugs, alcohol and women. Thus, his father now fears the worst.
Henry's first thought is to enlist the help of Hester, since her work in the East End clinic might prove useful. However, when he calls in at the clinic, the first person he meets is their old friend, Squeaky Robinson, who says he mustn't involve a good woman in this kind of search, and volunteers himself instead. Henry agrees and, joined by Crow, a young doctor at the clinic, the pair trawls the pubs, brothels and opium dens of the West End, which in some ways are even worse than those in the East. After all, these establishments cater to people with money, and as a result the entertainment procured is more extreme and dangerous.
The trio finds a young barmaid who actually knows Lucien and who insists on coming to help them, and through their sleuthing, they find even more troubling information about the missing youth: he had become obsessed with a woman called Sadie, and had begun working for a major drugs dealer.
Eventually, the group finds Lucien on the streets, but badly injured following a vicious knife-fight. He asks them to let him die: he's beyond redemption. But of course, they refuse to do this and instead patch him up and nurse him, all-the-while still wondering about the elusive Sadie. The only clue to her whereabouts seems to be a pool of dried blood.
Eventually they learn Sadie's sad truth--a tale of prostitution and murder. But just as this comes to light, the detectives' own situation becomes increasingly dire. The drug baron is not a man to be toyed with and Rathbone and his friends quickly realize he has them trapped, confined to a labyrinth of underground passages and alleyways.
Finally, with nothing left to lose, Lucien embarks on an act of bravery and self-sacrifice that will ultimately save all their lives and reunite father and son for a much-deserved Christmas celebration.
From the Hardcover edition.