There are plenty of terrifying baddies in the Sherlock franchise, but none can hold a candle to Moriarty. The baddie was introduced as one of the earliest villains to join the BBC show, and Moriarty has continued to be the worst. The psychopath singlehandedly gave Sherlock Holmes the kind of nightmares that pushed him into a drug relapse, but the final episode of season four introduced a villain of equally terrifying measure.
So, naturally, the two baddies were in cahoots.
During the finale of season four, fans learned that Moriarty had been aided by and partnered with Sherlock's sister Eurus. Audiences were shown a flashback of Moriarty entering a high-security prison known as Sherrinford. The secret location housed some of the world's worst criminals or controllable threats, and it was home to Eurus for years. The unstable genius demanded to meet with Moriarty for five minutes of unsupervised conversation as thank you for her helping predict three terrorist attacks in Britain.
So, while Mycroft would have rather done anything else, the solemn brother was forced to let the two converse. When Moriarty asked what Eurus could possibly have on Sherlock, the girl simply said 'Redbeard.'
Of course, fans later learned how the world fit into the overall finale. While fans knew that Sherlock's childhood dog was named Redbeard, they discovered the dog was little more than an elaborate traumatic hallucination. Rather than a dog, Sherlock's childhood best friend was killed when Eurus drowned them because of her jealousy towards the boy. Mycroft saw how much the ordeal affected Sherlock, and he spent years rewriting Eurus from his memory.
Once Moriarty passed, Eurus became obsessed with finishing her antagonization of Sherlock, and it all ended with a very high body count.
You can check out Sherlock's summary of the Season 4 finale below:
"In the final episode of this series, written by Mark Gatiss and Steven Moffat, long-buried secrets finally catch up with the Baker Street duo. Someone has been playing a very long game indeed and, alone and defenceless, Sherlock and Dr Watson face their greatest ever challenge. Is the game finally over?"
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Benedict Cumberbatch returns as Sherlock Holmes, with Martin Freeman as John Watson, Mark Gatiss as Mycroft, Rupert Graves as Inspector Lestrade, Una Stubbs as Mrs Hudson, Amanda Abbington as Mary Watson, Louise Brealey as Molly Hooper and Toby Jones as Culverton Smith.
Currently, there's no word on whether Sherlock will be renewed at any point for a fifth season.