The Doctor has pushed the Controller to the limits, and seen him dragged off by a claw. The trio of The Doctor, Jamie and Polly are ordered to work in the danger zone of the pit. The Doctor assumes the role of Supervisor, and works out a formula, the theory of which worries the authorities. Jamie manages to escape from the pit, hearing something he goes to investigate, and encounters a Macra. The Doctor aims to discover why the gas is being mined in such large quantities. Ben fights against the voices that are controlling him. Polly is terrified for Jamie as gas pours into the mine, the Doctor understands the Macra's reasons for diverting gas into the shaft. Jamie makes his escape from the slumbering Macra, but runs into another.
More serious in tone then the opening duo of episodes, a more sinister storyline which sees the crew placed in real danger. The charm of the opening episodes is lacking a little in the third segment, but it's enjoyable enough.
Worth a listen. 7/10 would be interesting to see how the Macra were realised.