|After Joan Watson and Mycroft are saved from being killed by Le Milieu, Mycroft brings Sherlock to meet Sherrington. As payment for saving Watson he wants Sherlock to solve a case. A former MI6 analyst, Arthur West, was murdered even though he was retired. Sherrington wants Sherlock to look into the case since the NYPD has it. Sherlock reports back and meets Walter who tells him that he knows "everything". Sherlock discovers that West's arms held the locations that a mole in MI6 visited and tells Sherrington. He's surprised and asks Holmes to join MI6 which Sherlock turns down and wishes him luck with the mole hunt. ("Art in the Blood") After Sherlock clear Mycroft's name and identified Sherrington as the mole, he approaches Walter who is the one who brought Sherlock on the West case. Holmes indicates Sherrington offered him a job at MI6 and Walter confirms the offer came from him. Sherlock accepts working for MI6. ("The Grand Experiment")
|Sherlock, with MI6's help, is able to trap and interrogate Vanja Borozan, an assassin with designs on his father Morland's life. Borozan commits suicide by jumping out a window. Sherlock meets Walter whose agents have collected Borozan's personal effects and taken care of the body. Walter comments that Sherlock has been asking many favors of MI6 lately but acknowledges that Holmes' time working for MI6 in 2015 gained him some chits. MI6 wasn't able to find who hired Borozan so Sherlock says he'll inform Sir Walter if he does. Walter then expresses his condolences on the death of Mycroft. ("You've Come a Long Way, Baby")
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