Sherlock/Shutter island AU:
Mr.Holmes is a highly delusional patient in a mental hospital.
Mr. Holmes is convince that he is 'Sherlock Holmes: the worlds greatest detective’ and there is nothing the doctors can do to convince him otherwise. Each doctor agrees that the only way to bring Mr.Holmes to sanity is to play along with his story. So they let him become 'Sherlock Holmes' by setting up crimes for him around London. The trustworthy, Doctor Watson is assigned to pretend to be 'Sherlock's' assistant whilst keeping an eye on him. Everybody that 'Sherlock' meets are all pretending to be part of his story. One of the nurses play a mortician, ‘Molly Hooper’; one of the male nurses play a detective of Scotland yard, ‘Greg Lestrade’. 'Sherlock's' brother, 'Mycroft' is played by the head doctor of the hospital and constantly worries about 'Sherlock', occasionally pulling John from his 'cases' to report back to him. Richard Brook is a hired actor to play the villain, 'Moriarty' in 'Sherlock's' story.
'Sherlock' solves crimes and helps people with the help of a few paid actors. However when the press catch a whiff of 'The great Sherlock Holmes' things start to get out of hand. Eventually John finds that the press have figured out that this 'Sherlock Holmes' is a fake loony and it gets published in the paper, despite the hospital trying to explain things.
After everything and everybody is exposed to Him, ‘Sherlock Holmes’ refuses to believe that he isn’t actually a detective and takes his own life thinking he is a fake.