We see Kitty eight months ago at Scotland Yard, working on her own to solve a a missing boy case. The police ignore her. She briefly runs into Sherlock outside.
Joan is worried about Kitty but doesn't believer her boss is guilty.
Sherlock counters that Del may have gotten worried when Kitty partnered with him and Joan. He asks Joan if anyone from Leda ever handled her phone. She says no, but admits it is a company phone.
Del calls and fires Joan. Says she was a great help.
Gregson and Bell interrogate Del Gruner. He denies knowing anything and his lawyer implies this is in retaliation for firing Joan.
They know he did it, but all the evidence here points to de Merville.
Kitty tells Sherlock that she is going back to London. She isn't ready to face Del.
Kitty texts to say she has just landed in London, except she never left New York. She watches Bruner arrive home.
Sherlock has identified several woman who fit Bruner's profile. Bell may have found a former target of Bruner's named Tabitha Baird. She claims it was a misunderstanding.
Kitty is recreating the nutmeg concoction recipe to dissolve a body. She tests it on a piece of steak.
In London, the boy, Latif, is returned to his parents. It wasn't the culprit Kitty believed it to be, but she was correct about the backpack being planted. Sherlock offers to teach her.
Joan attends a Hearth and Home fundraiser that Del is speaking at. He advises her to not make a scene and she tells him not to get comfortable.
Sherlock is upset that Joan confronted Del and gave him the names of the women they think he killed. Joan says he only reacted to Tabitha's name. Sherlock realizes that it's not Tabitha he is interested in, but her son.
They meet with Tabitha and she admits Jesse is adopted. They believe one of his captives became pregnant and Jesse is his son. She lets them take a DNA swab.
In London, Sherlock teaches Kitty how to open chain locks. They argue and she tells him not to contact her again.
The police search Del's home, but he is not there. Sherlock observes a door with a chain similar to the one he taught Kitty how to break.
Kitty tells Del that he will die horribly. There is a door knock and he yells for help. She hits him and opens the door. It's Sherlock.
Sherlock won't stop her from killing Del if it makes her whole again, but tells her they have a way to expose him for who he really is.
Kitty decides not to kill Del but she is going to take his mask off.
Sherlock arrives home and Joan is waiting for him. She asks if Kitty is okay. Gregson calls and says they found Bruner. His face has been badly burned by some corrosive. Gregson says he will ask him who did this to him and have to go after that person.
Kitty calls Sherlock. She is leaving but thanks him. She is going somewhere she can help people and tells him that she loves him.
In London, Sherlock is despondent. Kitty comes over and asks to restart her training. He agrees and throws away the heroin he had been contemplating.