The Famous Four - Part 2 - S2.E17

While Cathy and Chris battle the ever-increasing Annihilator forces in the present, Danny and Sam are in the past trying to alter the course of the future and prevent Mark from ever getting his hands on the tape. However, their efforts keep failing and, back in the present, the Annihilator mother ship has arrived with its Death-Star-style beam to flatten the town.