The taking of Chelsea 426

The Chelsea Flower Show - hardly the most exciting or dangerous event in the calendar, or so Doctor thinks. But this is Chelsea 426, a city-sized future colony floating on the clouds of Saturn, and the flowers are much more than they seem. As the Doctor investigates, a familiar foe arrives, and the stakes suddenly get much higher. The Sontarans have plans of their own, and they're not here to arrange flowers...