The question all along, it seems, was not ''who' is Sherrinford?', but rather ''what' is Sherrinford?'. The season four finale of 'Sherlock' entitled 'The Final Problem' was rife with plot twists and a story much darker than any previous episode. Plus, the return of Jim Moriarty - sort of. Warning: Spoilers ahead.

SherlockBenedict Cumberbatch and Martin Freeman take on their final challenge of Sherlock season 4

As it's revealed that Watson (Martin Freeman) was not, in fact, shot in the head with a bullet but with a tranquiliser at the end of 'The Lying Detective', Sherlock (Benedict Cumberbatch) and Dr. Watson confront Mycroft (Mark Gatiss) about the so-called Holmes sister Eurus (Sian Brooke), who Sherlock has no memory of. Mycroft eventually explains how she was their psychotic younger sibling whose genius far exceeded each of theirs. She was sent away at a young age after apparently killing Sherlock's dog Redbeard and is now held at a high security institution in the middle of the ocean called Sherrinford.

After narrowly escaping a motion sensitive drone carrying a grenade that decimates 221B Baker Street, the three of them set out to the island to discover how Eurus managed to escape the fortress which holds her to torment Sherlock and Watson in the previous episode. It turns out the answer is easy; her persuasive personality has infected the mind of the governor and therefore there is no security on the island. Eurus is running Sherrinford now. 

Sherlock, Watson, Mycroft and the governor wake up to find themselves trapped in a cell, as Eurus proposes a sick game involving the group being forced to make decisions in a series of morbid moral dilemmas. Their motivation is a phone call from a little girl on a plane, who is about to crash as every other person on the plane is asleep including the pilot. They must complete each task if they want to help the girl.

The tasks involve Sherlock choosing either Mycroft or Dr. Watson to shoot the governor, solving a murder with the twist that the suspect gets dropped into the ocean, getting Molly Hooper (Louise Brealey) to confess her love for Sherlock and, finally, trying to get Sherlock to shoot either Watson or Mycroft - all while Eurus played creepy recordings of Moriarty (Andrew Scott) as if he were in the room with them. She'd been planning this routine for five years. Unfortunately for Eurus, by the last task Sherlock doesn't want to play fair and attempts to shoot himself but all three of them are tranquilised yet again.

Sherlock s4Mark Gatiss and Benedict Cumberbatch return for season four

When Sherlock wakes up, he's back at his childhood haunt where Eurus drowned his dog. Elsewhere, Dr. Watson is trapped in a well that is quickly filling with water. To save Watson, Sherlock must work out what happened to Redbeard, and it's only when Watson informs him that the bones at the bottom of the well belong to that of a child rather than a dog that Sherlock realises the Mycroft has been doing him a kindness all along.

Meanwhile, the girl on the plane? It was Eurus talking all along in a state of psychosis, and Sherlock finds her looking deeply distressed in her old room at the house after revealing that she never had anyone to go to as a child. And even after he discovers what she did all those years ago, he still comes to her aid and decides that she'd never be alone again.

More: 'The Lying Detective' recap

There's a lot of love in this episode. When Mycroft didn't feel comfortable talking about Eurus in front of Dr. Watson, insisting that it was a family matter, Sherlock bellows: 'That's why he stays!' as Watson barely conceals his grin. Then later, when Mycroft insists on Sherlock shooting Dr. Watson instead of him, it doesn't take long for this sleuth to realise that his older brother is deliberately trying to make it easier for Sherlock to shoot him. 

It could be argued that the ending is rather gratuitous; 221B Baker Street is almost perfectly salvaged, and the entire Holmes family have reconnected with Eurus. It's a bit 'and they all lived happily ever after', but after that season, it's truly a relief.