Back in 1979/80, with Tom Baker as the 4th Doctor, there was a strike at the BBC, which caused the last story of Season 17 to not be completed. It was called "Shada," and was written by Douglas Adams (The Hitchhiker's Guide to the Galaxy). There have been previous attempts to present the story somewhat in the way it was intended; on VHS (which I have), with Baker providing narration for the scenes never filmed; an audio drama; limited animation for BBC online; and novelization in 2012 by Gareth Roberts. Sometime last year, a new version was completed.
Other episodes that had been lost had been recreated with some animation when the original audio was still available. In the case of Shada, since some episodes had not been filmed at all, the original cast were brought back to record the audio from the original script, with new animation to fill in the visuals. It has already aired and been released on disc in the UK, and is scheduled for DVD & Blu-Ray in the US in November.
It will air tomorrow night, Thursday, July 19, on BBC-America, running from 7pm-10pm CENTRAL time. I can watch it on SlingTV, which I probably will, since I don't know if they will offer it On Demand later. Every other story that year had four episodes, and I assume that was what had been intended for this one too, and I think they were 30 min. each that year. That means a lot of commercials, or maybe some bonus info between segments.