Despite a lengthy wait for the fourth series, Benedict Cumberbatch's Sherlock remains a proper TV event.

Not even 2016’s shaky New Year’s Day special – shall we pretend the whole Victorian dream sequence thing didn’t happen? – has dampened expectations for three new episodes of sleuthing and dark conspiracies.

Writers Steven Moffat and Mark Gatiss continue to mesmerise viewers with their breakneck scripts, dazzling visual flourishes and nudges and winks to the classic Conan Doyle stories sprinkled throughout.

Very loosely based on Doyle’s The Six Napoleans, the first episode back began as a cosy comedy, brushing aside Moriarty’s message from the grave (He’s still dead, they promise...), suddenly morphed into a Jason Bourne action movie with swimming pool brawls and fatal globetrotting showdowns before it all concluded with a heartbreaking family tragedy.

Sherlock

That doddery secret service secretary was more menacing and ruthless than anyone could have imagined.

Amanda Abbington’s Mary Watson was shot dead, leaping in front of Sherlock and taking the bullet to save him. The former secret agent’s past had caught up with her in the episode and although her exit was expected by many fans, the fall-out will dominate the rest of series four.

Here are the six talking points and burning questions we have after watching The Six Thatchers:

1. Did Mary steal the show or spoil it?

Sherlock

Whether you loved or loathed The Six Thatchers appears to depend on where you stand on the character of Mary Watson. The initial response to the episode on social media appears to be divided into two firm camps.

Those who were dazzled by Mary’s action-packed adventure, secret past and heartstring-tugging farewell. And on the other side, viewers who miss the days of Holmes and Watson solving murders as a double act rather than chasing after action hero Mary Bourne.

Although Cumberbatch’s Sherlock has always been about much more than the whodunit/howdunnit format of the original Doyle stories, with only a couple of episodes every other year, it’s easy to understand why some fans feel hard done by when episodes become diverted by increasingly bizarre sidesteps.

The writers seem to be making up for the scarcity of Sherlock episodes by rattling through three or four possible episodes in a 90 minutes. The breakneck speed of the show is one of its winning features, but perhaps its been guilty of late of pushing itself just a little too far.

2. Can Watson and Sherlock recover from Mary’s death?

Mary’s pre-recorded message for Sherlock insisted that he must “save John” should she ever die. However, their friendship appears to be in tatters at the end of this week’s episode.

John is a broken man. Even the usually cold and calculated Sherlock is a broken man. How they both recover from this and rediscover their friendship will be central to the remaining two episodes.

Sherlock

3. Who is John's bus woman?

The curious sub-plot involving John texting an ever-so-slightly creepy woman he met on a bus felt like it was added to the story at the last minute. Was it merely there to add to John’s pain and guilt in upcoming episodes? Or will she have a role to play in the next two instalments?

We suspect we haven’t seen the last of her and we don’t like the cut of her jib one bit.

4. Is Moriarty alive

Of course not, he’s definitely dead. We saw him die. Sherlock says he’s dead. Moffat and Gattis have insisted that he’s dead.
However, for as long as Sherlock and viewers have to wait to discover the truth behind the ‘Miss me’ messages, there will always be the slightest possibility of Andrew Scott’s dastardly villain returning.

We’ll probably have to wait two more weeks to find out for sure if he’s 100% brown bread.

5. Oodles of Conan Doyle references

One of the joys of Sherlock for crime fiction fans is picking out the references and nods to the original Doyle mysteries. This evening’s episode was overflowing with literary nudges to parts of the original canon such as the Giant Rat of Sumatra (Sherlock’s rewriting of the fable Appointment at Samarra), the Black Pearl (It really was the pearl hiding in the statues in the original text), AGRA (a treasure in Doyle’s The Sign of Four) and Norbury (a mistake Holmes made in The Adventure of the Yellow Face).

However, the key secret to the new series may lay in Mycroft’s call to ‘Sherrinford’ later in the episode. Fans believe that Sherrinford may be a third Holmes brother, who will be unveiled later in the series.

A third brother never appeared in Doyle’s original stories, but many Doyle experts have proposed that another sibling may have existed and created the name Sherrinford for the third brother.

6. Can Toby Jones silence the show’s doubters?

Sherlock

In all the teasers and previews for series four, Toby Jones’ villain Culverton Smith has featured heavily. He’s been described by Steven Moffat as one of Doyle’s “finest villains” and there was even a little Easter Egg reference to him in tonight’s episode with the character’s image showing on a bus stop poster.

Viewers were divided about this evening’s comeback episode, but we suspect the always fantastic Jones could silence any doubters as a cackling and terrifying new foe for Holmes and Watson.

Sherlock continues on Sundays at 9pm on BBC One. Missed it? Catch up on iPlayer and BT Catch-Up.

[Read more: Arthur Conan Doyle – 5 things you never knew about the Sherlock Holmes writer]

[Read more: Sherlock series 4: Benedict Cumberbatch teases the “darkest” series yet]