From the BBC:
Fury from the Deep is the missing sixth serial of the fifth season of Doctor Who, which was first broadcast in six weekly parts from March to April 1968. Starring Patrick Troughton as the Doctor, the story concerns a colony of sentient, parasitic seaweed, last seen in the eighteenth century, returning to attack a number of gas instillations in the North Sea in an attempt to take over humanity.
No full episodes of this story exist within the BBC archives, and only snippets of footage and still images are still around to represent the story. However, off-air recordings of the soundtrack do exist, making the animation of a complete serial possible once again.The six new animated episodes are being made in full colour, in the original black and white and in high definition and the release will include the surviving clips from the original 1968 production. It also stars Frazer Hines as Jamie McCrimmon and Deborah Watling as Victoria Waterfield.The serial will be released on DVD and Blu-ray next year.