Sherlock and Joan join Gregson and Bell in investigating the triple homicide of members of a Chinese triad. Realizing that the gangsters were lulled into a false sense of security, they initially suspect a fellow gangster had been subverted by a rival triad. They soon realize that the killer was actually a white man disguised as an old Chinese woman.
They link the case to an attack on another man and eventually realize that the Good Samaritan who reported the attack was actually the killer, Sven Ecklund, who works as a mortician.
When arrested, he readily confesses, but refuses to give a motive. The team discovers that he was attacked by the Triad at the behest of the owner of a nearby assisted living facility, who was working with the Triad. Sven had discovered that the facility had killed at least one of its residents to acquire the inheritance for the Triad. After Sven was diagnosed with a terminal ailment, he decided to take revenge on those who had hurt him.
Meanwhile, Sherlock continues to investigate the attempt on his father's life and turns his focus on Sabine, his girlfriend who died in the incident. Sherlock learns that someone was monitoring her email account, which was how they knew where she and Morland would be that day.
Morland also reveals that Sherlock's mother was, like Sherlock himself, a drug addict, and that her addiction was what led to their separation and divorce and ultimate death, for which Sherlock still blames Morland.