<P> While the production does not use linking elements, the show has included allusions and Easter eggs from previous episodes into later ones . For example, "Hated in the Nation" in the third series calls back to the events of both the first series episode "The National Anthem" and the second series episode "White Bear". while the final episode of fourth series "Black Museum" includes references to each prior episode of the series . Some of these are for the ease of reusing a name developed in a prior episode; for example, they created the fictional "UKN" broadcast network for handling news reports that drive stories, while the pizza delivery company used in "USS Callister" was used again for a company running automated pizza trucks in "Crocodile". Other Easter eggs were added as they knew viewers would be intensely dissecting the episodes and provided these as gags . However, over time the use of Easter eggs became more purposeful, as to establish a canon of the "dream universe" that the episodes take place in . Brooker noted that when they opted to reuse a cover of Irma Thomas' "Anyone Who Knows What Love Is", first introduced in "Fifteen Million Merits", for "White Christmas", as "it does sort of nest the whole thing together in some kind of artistic universe". </P> <P> Giles Harvey comments in a profile of Brooker for The New Yorker that each episode "establishes the background of normality against which a decisive tweak will stand out all the more starkly". Harvey notes that the show's diverse range of genres show that it is "manifestly the work of someone who has clocked up many hours of screen time". He further comments that Brooker is "scrupulous", as "the believability of each episode depends on maintaining the complex internal logic of its dystopic world". Brooker is involved during the filming and editing processes, pointing out any inconsistencies that arise, and is "determined to make the devices and screens and interfaces used in' Black Mirror' seem authentic". As examples of Brooker's "meticulous attention to detail", Harvey reports that Brooker carefully considered whether a falling wine bottle would shatter in the nested virtual realities of "Playtest", and whether it would rain in the eponymous location in "San Junipero". An instance of realistic technology is the email system in "Be Right Back", which contrasts with "histrionic computers" found in Hollywood; an email is sent to the main character with the heading "Martha, people in your position bought the following", containing various books on the topic of grief - counselling . Knowing that fans of the show have dissected some of these details by watching the episodes frame - by - frame, Brooker and his team have included humorous jabs at these fans through printed messages on various props, such as a paragraph in a news article held up by a character directed to fans of the show on Reddit in "Crocodile". </P> <P> The first two series of the programme were produced by Brooker's production company Zeppotron, for Endemol . An Endemol press release described the series as "a hybrid of The Twilight Zone and Tales of the Unexpected which taps into our contemporary unease about our modern world", with the stories having a "techno - paranoia" feel . Channel 4 described the first episode, "The National Anthem", as "a twisted parable for the Twitter age". Black Mirror series 1 had a limited DVD release for PAL / Region 2 on 27 February 2012 . </P> <P> A trailer for the second series was made by Moving Picture Company, and featured three interspersed storylines: "a dream sequence, the repetitive factory setting and the huge dust cloud that sweeps through the street at the ad's climatic end ." Aired from 22 January 2013, the advertisement was shown on Channel 4 and in cinemas . The second series had a DVD release, which like the first series DVD was PAL for region 2 only . </P>

What episode is the beginning of black mirror