In London, England, the funeral of Warren Pendry is held. The funeral is interrupted by Gareth Lestrade who yells insults at Warren. The priest tells him to leave the private ceremony but Lestrade wants to pay his respects. He holds a grenade in the air and accuses Warren's son of murdering his wife, Mary. Warren ruined Lestrade's career as a police officer. Lestrade speaks to Warren and says he is wrong to think that he ruined Gareth.
Watson meets Sherlock in Washington Park. He realizes that three attorneys have been murdered in a case prosecuting a syndication of pirates. They believe a man named Jeremy has been hired to commit the murders but there was no proof of him contacting the pirates. Sherlock went to Jeremy's apartment and found a carrier pigeon with a message on it. He is waiting to see who comes to collect the message. They watch as a man catches a pigeon. He sees Sherlock walk toward him and he takes off running. He manages to elude Sherlock but Watson catches him.
After the police take the man away, Sherlock answers a phone call from an unknown number in the United Kingdom. After the call he asks Watson if she has a current passport. They need to go to London, because his old partner Lestrade is in trouble.
Sherlock has never mentioned Lestrade to Watson before now. Lestrade was exposed and Sherlock feels like it was his fault. Watson asks how Sherlock will feel returning to London because he left in disgrace. He says that he has changed and so has London.
When they get to London, Watson suggests that now is a good time to work on step 9. They meet up with Hopkins, an old colleague of Sherlock's. Lestrade accused Lawrence Pendry of killing his wife Mary. Hopkins explains that the police never found a gun so Lawrence had to dispose of the weapon within the eight minutes that it took police to get there. After Sherlock left for the states, Lestrade had a hard time closing cases and Pendry shamed him and New Scotland Yard in the papers. Hopkins reminds Sherlock that he isn't there to advise on the case of Mary's death but to find Lestrade.
Lawrence Pendry enters the station to meet with Hopkins. He recognizes Sherlock as the mastermind behind all of Lestrade's successes. Lawrence wishes that Sherlock was with Lestrade the night Mary was killed. Maybe then Mary would be alive and Lestrade would still have his job.
Sherlock takes Watson to his old home, he has a friend who maintains it for him while he is in America. He says he spent 10 years making it perfect. He enters the apartment and there aren't any of his belongings. While Sherlock looks around upstairs a man enters and asks Watson what she is doing in his place. Sherlock comes down and sees the man. He introduces Watson to Mycroft Holmes, his brother.
Watson asks why Sherlock never mentioned Mycroft and Sherlock says they are only similar genetically. Mycroft is nice to Watson and says he may know why Sherlock didn't mention him. Sherlock cheated on Mycroft's girlfriend because he thought she was only dating him for money. Mycroft says that while they have their differences they are still family. He invites both of them to stay in the guest rooms for the night. Sherlock leaves because he knows how to find Lestrade.
Sherlock heads to a bar and finds Lestrade. Lestrade asks how he found him and Sherlock explains that he had 5 caches around town in the case of an emergency. Four of five caches were already emptied and the fifth was nearby. Sherlock tells Lestrade to come with him to Scotland Yard or he will bring the Yard to Lestrade. After they leave the bar, Lestrade makes a go for the last cache. He asks Sherlock if he will work with him one last time.
The next day Mycroft tells Watson that Sherlock never came back last night. Mycroft has a hard time believing the two are friends and doesn't believe that he has changed. Sherlock invites Watson to an abandoned theater that Lestrade is hanging out in. Watson asks why Sherlock hasn't turned him in. Sherlock has agreed to help Lestrade investigate Mary Pendry's murder. Watson tells Sherlock that Mycroft wants to take her to dinner and Sherlock thinks Mycroft wants revenge on his brother.
Sherlock hatches a plan to convince Lawrence Pendry that Lestrade wants to kill him. This will allow Sherlock to get inside the house without raising any questions. After they visit Pendry, Sherlock believes that he did kill his wife and he knows how.
Back in Lestrade's theater, Sherlock says that the reason the case against Lawrence failed is because there was no weapon found. Lestrade says they searched everywhere for a gun but Sherlock says the weapon was a plastic gun. Sherlock believes that Pendry placed the gun in a bottle of acetone in the fridge. To Lestrade it looked like a bottle of milk but Pendry was lactose intolerant. The gun was 3D printed and only needed one piece of metal for the firing pin. Sherlock states that Pendry used a nail and since he couldn't melt it down he used it to hang a mask on the wall. In Pendry's house, Sherlock inspected the nail and says it had a blackened tip. Lestrade wants to go to Scotland Yard now but Sherlock doesn't think they have enough evidence. He needs to find how the gun was made.
Sherlock knows a man who can find the 3D printer but the man has no phone. He stands under a security camera with a sign that has a message for Lang dale Pike. He tells Watson that all he has to do now is wait. They end up waiting four hours until a man bumps into Watson. Sherlock says it was Lang dale Pike and he dropped a list of people, who have bought 3D printers, off to Sherlock.
Watson meets Mycroft for dinner and tells him nothing will happen. He says that it wasn't supposed to be a romantic date. Mycroft tells Watson that he lied about losing weight, he was actually sick. When he was sick, he realized that his one regret was the state of his relationship with Sherlock. Mycroft says that Watson is Sherlock's first friend and he wants to know how he can be friends with him too.
Sherlock apologizes to Lestrade for not protecting him from the spotlight. Lestrade says it was the best time of his life but Sherlock still feels bad. He wants to make amends and he wrote some things to say. Lestrade stops him though because he has found the man who he thinks Pendry paid to buy the 3D printer.
They visit Nick Gin, the man that bought the 3D printer. There is no answer at the door so Sherlock picks the lock. Lestrade says he knows how Sherlock can make amends with him. If Sherlock will let Lestrade take the credit for this case he will consider them even. Lestrade breaks the door open and they find Nick. He lies dead on the floor with a knife stuck in his chest.
At the crime scene, Watson points out that it is strange that Pendry would use his non dominant hand to stab someone and also come without a weapon. The knife was already in Nick's apartment when Pendry arrived. Sherlock notices a small hole in an apple and bites it open. He says he was wrong about Pendry no longer owning the 3D printer because he has used it to build another gun.
Sherlock, Watson, and the police arrive at Pendry's to arrest him. Sherlock says that the gun exploded and so he had to use his left hand to stab Nick. The police arrest Pendry and Sherlock meets Lestrade outside the house. Lestrade thanks Sherlock but Sherlock says he won't allow Lestrade to take credit. He no longer wants to be an enabler.
Later on TV, Lestrade calls Sherlock's bluff and takes credit anyway. Mycroft texts Sherlock and says he urgently needs to meet with him. Mycroft confesses that he lied about giving Sherlock's stuff to charity, it is held in storage. Sherlock senses something wrong because Mycroft is too calm. Mycroft received advice from Watson about how to befriend Sherlock. She told him to ensure Sherlock is listening. He detonates a bomb and a blast shoots out of the storage where Sherlock's things are. Mycroft says they are even and now he can forgive his brother for everything. He wishes him a safe trip back to America and tells him that things are different between them now.
Sherlock meets up with Watson and says that he has reconciled with his brother. He tells her about the home made bomb that Mycroft used to blow up the remainder of his things. Watson believes that Mycroft is more like Sherlock than he thought. Sherlock says art in the blood takes the strangest forms.