This is Victor Pemberton's adaptation of his own script, which is the final story of companion Victoria.
Landing on the coast of the North Sea in England, where the first indication that something is wrong is a strange, rhythmic sound the Doctor can hear from inside a gas pipe. Before they can investigate. They are shot with tranquilising darts and wake to find themselves accused of sabotage. The crews of some gas rigs have vanished, and the operations of refinery have been subject to odd changes in gas pressure within the pipes.
What is it lurking in the refinery's pipes?
This is one of many stories of which the BBC no longer has a copy. They released the soundtrack on audio cassette some years back, and it is likely to be re-released on CD sometime in the future. Some clips of the series can be found on 'The Missing Years' video, released in conjunction with 'The Ice Warriors'.
While Mr. Pemberton's novelisation is quite good, and benefits from its higher than normal page count, watching the few remaining clips only emphasises what has been lost. The portrayal of Mr. Oak and Mr. Quill in particular loses something without its visuals.
This story is one of those that does not make the transition from one medium to another without losing something.