Fans left reeling by shocking twist in Inside No 9's The Stakeout

Thompson is shaken, having recently lost his previous partner in a violent attack, the circumstances of which are gradually revealed over the course of a tense half-hour.

The truth comes out that Varney is a vampire and turned Thompson's former colleague into an undead monster, who makes a frightening return at the end of the episode.

Inside No 9 is a show built on jaw-dropping twists, but fans were particularly taken aback in this case, praising a script laden with hints and clues - including the title itself.

In the episode, the two officers play a word game in which two people improvise a story, but can only start sentences with the words fortunately and unfortunately.

This format was adopted for a number of fan tweets, including one from Detectorists star Rebecca Callard, who had a small voice role in the episode.

On Monday, the BBC announced that Inside No 9 would be returning for another two series, each consisting of six new episodes.

Series creators Reece Shearsmith and Steve Pemberton said: "We are delighted and exhausted in equal measure at the prospect of two more series-worth of tall tales and fearsome fables. However, we will endeavour to showcase the very best of what the BBC can do in every genre with 30 minutes of story-telling and some half-decent wigs."

No premiere date has been announced for the new episodes, but given the two-year gap between series four and five, it would be wise not to expect them too soon.

Inside No 9 is available to stream on BBC iPlayer