Warning: MAJOR SPOILERS ahead for Sherlock season 4: 'The Six Thatchers'

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Over 8.1 million U.K. viewers tuned in to the highly anticipated return of Sherlock season 4, with the first episode titled "The Six Thatchers."Ostensibly, the episode dealt with Sherlock discovering who was breaking into properties and destroying busts of the late Margaret Thatcher, but (as ever with Sherlock) it was the subplots that were far more interesting, with one thread of the storyline in particular becoming not only the main focus of the episode, but also of the entire series from now on.

When Mary Watson threw herself in front of a bullet meant for Sherlock, she did so in a split second decision that ultimately cost her her life: sacrificing herself for the man who had sworn to always protect her, no matter what. In Mary's former her life as a secret agent, she had watched too many of those she cared for die at the hands of another - in fact, earlier in the episode, A.J, her fellow assassin from a former case, had been shot right in front of her. It's surely little surprise that she was so protective of Sherlock, her husband's best friend and one of the few constant supporters in her life. As Mary lay dying, she apologized to Sherlock for having shot him once before, and claimed that her being shot this time around made them even.

However selfless the act of saving Sherlock may have been, it was a cruel twist of fate for John, who now finds himself a widower and single father to a small baby. For much of 'The Six Thatchers,' viewers watched John struggle with fatherhood coupled with the added complication of having Mary as his wife. It later became apparent that he had embarked upon (at the very least) an emotional affair with another woman. Maybe his ego felt threatened by Mary's past, maybe he was bored of being a dad, maybe he was feeling undesirable... or maybe that subplot is actually a total misdirect on the part of the showrunners.

The turmoil John was going through as he cradled Mary in his arms was evident, thanks to a sublime performance by Martin Freeman, and we've yet to really see how destructive an effect Mary's death will have on John's relationship with Sherlock. After all, the shooting probably would never have happened, had Sherlock not let his ego (and mouth) run away with him. In the immediate aftermath, we saw John angrily tell Sherlock to keep away, and Molly later reiterates this message when Sherlock attempts to visit. So... what now?

Sherlock is more affected by Mary's passing than fans have ever seen him before. In fact, so desperate is he to mend the rift with John that he sought help in the form of a therapist. Add to that the impact of Mary's posthumous video message, and it's safe to assume that Sherlock - saddled with guilt and misery - will stop at nothing to get his friend back on side.

John has had to forgive many things from Sherlock before, including (but not limited to) his disappearance for three years following his staged death, but the loss of Mary may be the straw that breaks the camel's back. A look at the second episode, "The Lying Detective," shows that John is, it seems, back on the case with Sherlock, but how will he manage to overlook his recent bereavement and Sherlock's role in it?

"The Six Thatchers" picked up very soon after the events of the final episode of season 3, despite the actual three year difference, and there is no suggestion that a large amount of time has elapsed between "The Six Thatchers" and "The Lying Detective." It's possible that enough time will pass to give John a chance to cool off a little, but it's more likely that in order for John to really understand how much he needs Sherlock (and vice versa), Sherlock has to be put in peril once more.

This peril will come in the form of Culverton Smith (Toby Jones), allegedly the darkest villain that Sherlock has ever encountered. The original Arthur Conan Doyle story has Sherlock pretending to be poisoned, with John desperately trying to find a cure. Sherlock sends him away and asks him to summon Smith, then hide. Thinking he is alone, Smith confesses to the crime. Whether all that will unfold on screen remains to be seen, but would Sherlock really be cruel enough to let John grieve for him again, so soon after losing his wife? Well, this is Sherlock, after all. Though he needs to (and is trying) to make amends, the self-proclaimed "high-functioning sociopath" has a funny way of going about things.

The events of "The Six Thatchers" and the fallout between John and Sherlock currently seems insurmountable, and Mary's death will no doubt alter their relationship irrevocably. Even if their relationship does get back on track, there will always be a lingering doubt surrounding Sherlock's reliability and trustworthiness that can never be overcome. If the air is still strained between the pair, maybe the elusive Sherrinford Holmes can help make it right?

Talk of the third Holmes brother has been rife since the clue for season 4's final episode "The Final Problem," was given as Sherrinford, and Mycroft alluded to there being more than two Holmes siblings in season 3. At the end of "The Six Thatchers," we saw Mycroft making a call, and asking to speak with Sherrinford. Given that episode 2 will focus on Culverton Smith, it seems as though if this elusive brother does make an appearance, it will be in episode 3, and he could possibly help build bridges between the two estranged friends.

Showrunnner Steven Moffat has told fans that the ending of "The Final Problem" will explain why there will be such a large gap before the next season comes to air... could it be that this time John is the one to trick Sherlock, leading him to think all is well between the two of them, only to later exact his revenge? John Watson as a villain would certainly be a dramatic development.

Sherlock season 4 continues with "The Lying Detective" on January 8th, 2017.