Picking up from where we left off…
John and Mycroft helped Mary fake her death. Sherlock wasn’t in on the plan.
John got hooked up to TD 12 to make him forget about the fake death–that’s why he was acting like Mary was truly dead in The Lying Detective.
Before John’s memory was erased though, he wrote a note to Sherlock and asked Molly to deliver it.
Whether by accident or design, something about John’s note gave away the fake death to Sherlock.
Sherlock Holmes needed his memory to be erased now, too.
Mary and Mycroft knew Sherlock wouldn’t be likely to agree to a memory wipe on his own, so they set their own plan in motion.
Mary left a video message for Sherlock encouraging him to engage in risky behavior in a bid to reconcile with John.
Mycroft and Mary then sought out Wiggins. The whole fake death plan was going down because they were trying to go up against bad guys known as the Scowrers (or that’s their name is in canon, they may well end up with a different one here). Wiggins used to be a Scowrer, and Mary knew it. So they were able to either convince or blackmail Wiggins into helping them out.
When Sherlock predictably went back on drugs as a way to mess himself up to the point of John not being able to resist helping, Wiggins was there to provide.
Remember Sherlock’s own words from His Last Vow, though: “Wiggins is an excellent chemist.”
See the “coffee” he’s pointing at? That’s an IV bag full of some brownish fluid.
Sherlock flashed to the same stuff briefly while he was throwing “Faith’s” gun into the Thames, because that moment reminded him of what he knew about the fake death. What the TD 12 had made him forget.
Wiggins mixed up some kind of drug cocktail with TD 12. Sherlock (knowingly) gets high and (unknowingly) gets his memory erased simultaneously, thinking it was his own idea the whole time.
(Another option for this is that the effects of TD 12 had proved too weak for Sherlock’s unusual mind, and Sherlock was instead upgraded to the stronger TD 13. In which case, that’s what the post-it on Mycroft’s fridge was referring to. But I’m going to keep calling it TD 12 until someone in the show acknowledges the existence of another version.)
The connection between the drugs and TD 12 was also foreshadowed when John checked Sherlock for signs he’d been using again by pulling up Sherlock’s right sleeve.
Eventually, Sherlock went so far off the deep end that Wiggins bolted from 221B.
Mycroft then brought a team of agents into the flat, claiming that he was trying to figure out “what drove Sherlock off the rails.”
The reality was Mycroft already knew exactly what had done it. But he did use the opportunity to feel out whether John had caught on yet.
Mycroft also had his agents “collect evidence,” which was really just an excuse to get the incriminating TD 12 / drug cocktail remains taken away.
Like it never even happened.