On Sherlock Season 4 Episode 2, we are introduced to a new villain, Culverton Smith (Toby Jones), an admitted serial killer who confesses to his "friends" under the influence of a memory-modifying drug. His daughter, Faith, is among those he confessed to.
Meanwhile, Sherlock has started using drugs again. While high, he gets a visit from Faith Smith, who tells him about her case. Sherlock deduces that she is suicidal, and decides to keep her company and walks through London with her all night.
John Watson has started seeing a new therapist in order to deal with Mary's death. Sherlock goads Culverton Smith online, and then sets up a scenario that allows him and John to be picked up by Culverton Smith's people.
Sherlock and John go to the hospital that Culverton owns, and Sherlock confronts him in the morgue. However, his plan to confront Culverton with his own daughter backfires when he realizes that the woman he spent the evening with was not the real Faith Smith.
Culverton convinces John that Sherlock is simply high and does not know what's real. Sherlock attacks Culverton, but is stopped by John who pummels him. Sherlock is admitted to the hospital.
John goes to Sherlock's house, and finds Mary's message to Sherlock, and realize that all of Sherlock's behaviour, drug use and this case were an attempt to give John someone to save. John rushes to the hospital, and finds Culverton Smith attempting to kill Sherlock.
John later keeps Sherlock company through his withdrawal, and confesses to cheating on Mary by texting another woman. Sherlock comforts him through his grief.
John goes back to his therapist, who reveals herself as the woman on the bus, the woman who visited Sherlock as Faith Smith, and as Sherlock and Mycroft's secret sibling.