Synopsis: Having scavenged some of the downed ships on the planet's surface, the Doctor finds an anti-grav pack and makes an easy descent deep beneath the planet's surface.
He's looking for his lost TARDIS. And he rather easily finds it. But he also finds an entire race trapped in the inhospitable climate. They're under threat, much as the TARDIS itself is, of a huge serpent.
The Doctor jumps into the mouth of the beast and, using his anti-grav pack, forces the serpent to extend vertically. This provides a neat leadder by which the area can be evacuated. The Doctor leaves his new friends on the surface, after having worked out that they're adaptive enough to survive there using the remains of the vessels strewn about the planet's surface.
He descends again to retrieve the TARDIS, only to find that it has been pulled even deeper into the planet . . .