The Awakening

The Doctor has promised Tegan that they will visit her grandfather in the English village of Little Hodcombe, in the year 1984, a precision of timing and location that the TARDIS has not always achieved... When the Type-40 machine comes to rest, the view on the scanner screen only serves to confirm Tegan's rather low expectations of the TARDIS's performance.

The most sensible course of action would be to leave immediately — but despite Turlough's protests the Doctor rushes out to take on a seemingly hopeless rescue mission...

Frontios

The TARDIS is dragged down to the surface of the planet Frontios and apparently destroyed during a meteorite bombardment. The Doctor is forced to help the planet's human colonists - refugees from a doomed future Earth - and eventually discovers that their problems stem from an infestation of Tractators, burrowing insect-like creatures led by the intelligent Gravis.

The Tractators have been using gravitational force to cause regular meteorite bombardments in order to keep the colonists weak so that they can prey on their bodies and use them as components in their mining machines.