Back in the '60s and '70s the BBC made a habit of wiping old tapes and disposing of used film, taking the only copies of TV programmes along with them. This was back before home video releases were a thing, and before people could make their own copies at home, so those programmes were often lost forever. Doctor Who was one of the most famous series affected, and in recent years the BBC has been working on recreating those lost episodes via animation - the latest of which will be released next March.
The story in question is 1967's The Macra Terror, which saw the Doctor and his three companions wind up at a relaxation resort that was secretly controlled by a race of giant parasitic alien crabs - the titular Macra. The Macra would later reappear in 2007's Gridlock hiding in the smog beneath New New York and pinching any drivers daft enough to venture within reach.
The original four episodes are seemingly lost forever, but because the audio is still around the BBC has got to work animating them. It'll be available to buy digitally, on Blu-ray, and on DVD from 18th March 2019, with episodes available in both colour and black and white. Pre-orders for the physical copies are open at Amazon and HMV now, with the DVD, Blu-ray, and Steelbook costing £13, £18, and £25 respectively. HMV is currently far cheaper than Amazon, but I would imagine Amazon's prices are placeholders that will eventually decrease to something far more reasonable.
If watching at home using your giant TV still isn't good enough, BFI Southbank will be screening all four episodes on 16th March 2019. Tickets are already open for BFI members, and will open to the public next Tuesday (12th December).