For those who are unfamiliar, this is Douglas Adams famously unfinished doctor who episode.
Seeing as it was unlikely to ever be released, Adams recycled the plot in to his novel Dirk Gently's holistic detective agency, replacing the doctor with the titular gently (and effectively making Dirk gently doctor who fan fic)
Then it was rereleased as a radio episode and a flash animation, and a fan novel, and a recut fanedit, and a VHS with linking narration, and an official novel, and now this.
Even excluding the original Dirk Gently novel, Shadaay be the easiest episode of doctor who to accidentally encounter.
I know several people for whom it is the only episode they have seen.
Adams was, shall we say, less than pleased with this turn of events.
But he was less than pleased with a lot of things.
I've written about this before: http://ajroach42.com/shada-douglas-adams-unfinished-doctor-who-arc-reborn-in-countless-works-of-fan-fiction/
I may update that post to note that it's been officially released now.