Mission Dalek (also referred to as #missiondalek by whichever poor sod's wired in to the @bbcdoctorwho Master Brain at the moment) was a radical time-saving and cost-cutting manoeuvre devised by Moffat, after a whopping budget cut from the execs and increasing pressure to actually work on that other shitty show he does (what's it called again? I forget). The core of the idea was that, lacking the ability to produce a full run of episodes for 2016, the team could effectively crowd-source the work to the show's exceedingly creative fanbase by putting up a huge amount of free source material online. Assuming each fan made a short of roughly 1 minute 30 seconds, it would only take 30 entries to amount to an entire single episode.
The plan hit a snag when the BBC accidentally wiped 70% of the rights-cleared footage and audio from their archives. An official statement from the Corporation claimed that it was the result of an intern's mishap whilst trying to purge all mention of Jeremy Clarkson. It was later noted that on the night of the alleged incident, unusual seismic activity was detected near the tomb of Patrick Troughton.
Further complications ensued when two of the remaining three backup drives were mistaken for pies by Colin and promptly devoured, leaving only 10% of the original resources intact. The remaining drive - containing by now the sole existing copy of In The Forest of the Night - was only secured by Barrowman hiding it up his rectum; both Phil Morris and Ian Levine claim the other retrieved it at some point and is keeping it from public access.
At this point Moffat said fuck it, scrounged a handful of leftover clips, images and audio from Youtube and fan sites, bunged them all up on the BBC website, and used the thin cover of the BBC's Make It Digital initiative to encourage fans 16-years-and-up to somehow technologically hack it into coherent viewing (or failing that, just animate it all themselves).
As one might expect, the only viable results of this were meme-drenched pisstakes, some of which even /who/ had a hand in.
Unfortunately, when it came to assembling the string of incoherent, flashy minisodes into a single narrative, not even Moffat (usually an expert at this) could manage it. This is speculated to at least in part be the cause of the 2016 hiatus.