This is a phenomenal story with a huge, unique universe threatening enemy.

Landing on Gallius Ultima, its very quickly discovered that something is wrong- a ship that had previously gone missing is now returning, and it's not slowing down as it heads towards the planet. This leads to a very tense first part where we're already threatened with imminent death.

The story only just keeps getting larger in scope. The Doctor and Tegan are sent light years across space to investigate what destroyed the Kepler. It results in alternate realities, different universes and a creature the likes of which the doctor has never seen before. Before long, the entire universe is threatened by the Star Men.

The side characters are especially great. Autumn develops a close bond with Adric and it leads to a pretty heartbreaking ending between the two in a dramatic cruel twist of fate.

This stands as one of my favourite Doctor Who stories ever.