• Benedict Cumberbatch, Boyd Hilton and Martin Freeman attend a screening of the Sherlock 2016 Christmas Special at Ham Yard Hotel on December 19, 2016 in London, England.

Benedict Cumberbatch, Boyd Hilton and Martin Freeman attend a screening of the Sherlock 2016 Christmas Special at Ham Yard Hotel on December 19, 2016 in London, England. (Photo : Getty Images/ Jeff Spicer)

The final episode of "Sherlock" season 4 was a mishmash of psychological torment, sibling rivalry and the introduction of one of the sickest villains to have ever been introduced on TV. But while the events were mostly nerve-racking, one moment gave fans some comic relief when a fan-favorite celebrity suddenly rolled up.

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Dressed as a Viking, actor Paul Weller made a quick appearance in the Season 4 finale to portray the unnamed man who brought Dr. John Watson to the 21st century. In real life, Weller is a good friend of Martin Freeman, who portrays Dr. John Watson in the series. Freeman also previously appeared in Weller's 2015 single "Pick it Up," according to Metro.

While the actor's cameo only lasted for a couple of seconds, his scene was significant in moving the storyline on following the re-institutionalization of Eurus, Mirror reported. In the episode, the psychotic sister of Sherlock was re-institutionalized after going on a murderous spree at the Sherrinford center.

After the traumatic events that happened to Sherlock, and after the lives of his brother Mycroft and friend John were finally dealt with, the end of the episode showed the titular character and his companion moving back to their newly renovated flat at 221B Baker Street.  The scene featured a montage showing the pair back to solving mysteries in their new flat, following the explosion that took place earlier in the episode.

The scene showed Sherlock and Dr. Watson reopening the doors of their flat to their clients, including an insentient man lying on the ground and dressed in leather and fur. As Watson saw the man, he crouched over him and checked if he was still breathing.

Towards the end of the episode, the voice of Mary Morsten was heard on top of the montage. She said, "When life gets too strange, too impossible, too frightening, there is always one last hope. When all else fails, there are two men sitting arguing in a scruffy flat like they've always been there and they always will. The best and the wisest men I have ever known. My Baker Street Boys: Sherlock Holmes and Dr. Watson."

Watch the promo for "Sherlock" Season 4 finale here: