The Doctor and Tegan are trying to take the Gravis to a new planet, but the planet on which they have landed isn't the right one. The Doctor wants to explore, and the Gravis is willing (at first) but Tegan isn't interested in this wet, muddy planet. The Doctor figures out what has happened. There are no mysterious enemies — the Queeth are trapped in a temporal loop that has the planet removing itself from the universe, over and over again. The Doctor takes the Gravis to Kolkokron, then goes back in time to take the surviving Queeth to Queth, to become their own ancestors.