The companions' struggle for survival against the hostile environment of Aridius, including Mire Beasts and the drying landscape.

In the claustrophobic confines of the tunnel, Vicki spots a Mire Beast blocking their path. Ian, reacting on pure instinct, hurls a rock at the creature to drive it back. …

The Doctor’s group is preparing to relocate to the main airlock when a Mire Beast violently breaches the walled-up doorway, seizing Barbara in its tentacles. The Doctor intervenes, freeing her, …