The episode begins and a woman comes home from a jog/the gym, gets a drink of water, and answers banging at the door. It's a "delivery guy" who wants to charge his phone. It's clearly a scam and she tries to blow him off. Coincidentally, as that guy walks away, another guy in a hoodie busts in and drugs her and drags her body into the kitchen. He then apparently puts her in a suitcase and wheels her away. The first guy is shown thanking a cab driver for letting him charge his phone with his charger in the cab. Sherlock in the meantime shares in a group about a crime, going on and on at length about a mongoose, and the group is clearly not amused.
Watson points out that it was first share and it had nothing to do with addiction. He claims as a people person he wanted to cheer up the group. A man walks out of their shower, naked. He's Rhys. He let himself in. He informs Sherlock that Emily has been kidnapped. Turns out, Rhys is Sherlock's former drug dealer. He's British and Emily is his daughter who lives in New York. He has been sent a video ransom note and wants Sherlock to help. Watson worries Rhys is a trigger. Sherlock says he is but points out that Rhys is no longer a drug dealer and he needs his help. Rhys later shows Sherlock the video of his daughter bound and gagged. The kidnapper wants 2.2 million dollars and he has four days to get it to him or she dies. Rhys has the money, which he stole from Dominican drug dealers and went into hiding.
He didn't know anyone knew about his daughter. He only has 2 thousand dollars left. Basically he's a gambler. Sherlock says they have 44 hours left and that's twice what he needs so they should be good. They go to his daughter's house. Watson is impressed with Rhys' sense of calm. He says he believes in Sherlock Holmes. Sherlock deduces how the kidnapping went down. He finds a trace of a hand stamp from a Dominican nightclub. They go there to find clues. Rhys looks for any familiar faces. He spots a guy in the VIP section who is one of the U.K. suppliers. Sherlock follows a guy into the bathroom. He asks the guy about the kidnapping. Turns out, the guy is a DEA agent. The guy slugs him and he claims the guy tried to steal his wallet. As he beats him up he explains that the cartel isn't behind the kidnapping.
He apologizes for having to do it this way and drops him to the floor. The next day, Sherlock reads Emily's Twitter feed. He's given up on the cartel but still thinks the abductor was in the club. Sherlock finds a tweet regarding her stepdad Derek and him asking her for money. He's broke and working as a parking attendant thanks to the crash. He gets this info from Det. Bell. They stake out the guy at his valet job. They follow Derek after he gets off work. He goes to one of the buildings he owned, with groceries. Sherlock thinks maybe Derek kidnapped his own stepdaughter. They go in and look for her. It turns out Derek is just squatting, he didn't know about Emily's kidnapping. He says he raised her and would never hurt her and she only helped him out in a pinch.
They leave him alone since Sherlock says he reeks of innocence. The kidnapper in the meantime calls Rhys and wants to know who he's working with. Sherlock assures him he's not a cop. Regardless the kidnapper has left them a package, Emily's finger. He says he has also shortened the deadline by six hours. Sherlock examines it and confirms it is hers, and thinks it holds clues to her location. It has a distinctive burn from a radiator in a pre-war building. He also finds remnants of food under the fingernail and thinks he can deduce where it came from. Sherlock again watches the ransom video since the restaurant idea didn't work out, as there are too many of them. Rhys advances his theory that cocaine helped with a case in the past, intimating that could work now. He’s got some cocaine.
He gives it to Sherlock and asks him to do it "for Emily". Sherlock throws him to the ground and screams in his face. Rhys says he needs his "meds" and says he's begging him. Watson comes down and Sherlock flees. Later, sitting on a park bench Sherlock calls his father. He then returns home and says Rhys will be reunited with Emily within the hour. He reached out to his father for a loan. His father wants him to perform 'errands' in return. He tells Rhys he got the money less to save Emily than to rid himself of Rhys and they are never to speak again after this. Sherlock goes to meet the kidnapper. The DEA agent who Sherlock crossed path with earlier comes to his house and says he has information about the girl. Sherlock runs from a group of people dressed as painters.
It is an ambush. He breaks into a house to escape and calls Watson and says there is only one person who knew he was investigating the case. Watson turns and it is shown the DEA agent has pulled a gun on her and Rhys. He ties them up. He tells Sherlock his friends are safe. They chat while Sherlock hotwires a car. Sherlock figured out that the DEA agent heard about Rhys when he went undercover and how the Dominican cartel couldn't find him or the money he stole. He then tracked down Rhys and planned the kidnapping to get the money. Rhys manages to get himself and Watson loose from their cuffs with a pocket knife. They try to run but the agent shoots him. As they struggle Watson knocks him out.
In the meantime, at the police station the agent claims he has no idea who Emily is and that he became aware of Rhys while undercover and claimed he followed him to Sherlock's house and he attacked him and had to restrain Watson who he assumed was working with Rhys. Gregson says he talked to Sherlock, and if Rhys pulls through he wonders who they should believe the agent says he doesn't care what a known drug dealer has to say. They tell him if he tells where the girl is the DA will make sure he gets protective custody. He says he can't help.
They are searching the buildings and say he better give it up before they find her on their own. He finally gives it up. Sherlock on the other hand goes to the hospital and leaves an envelope for Rhys. He wakes up. Sherlock tells him that Emily is safe. The envelope is full of cash so Rhys can leave. Sherlock's own, not his dad's, which he returned. Rhys again says he believes in Sherlock and always will. Emily comes in and hugs him. Sherlock later admits to Watson that Rhys offered him cocaine and explains the drugs/epiphany theory. He didn't use it. He wants to share the story at a group meeting. She is impressed and they go to the meeting. The episode ends at this point.