The Chelsea Flower Show - hardly the most exciting or dangerous event in the calendar, or so the 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, he becomes more and more worried. Who is shopkeeper Mr Pemberton acting so strangely? And what is Professor Wilberforce's terrible secret? They are close to finding the answers when a familiar foe arrives, and the stakes suddenly get much higher. The Sontarans have plans of their own, they're not here to arrange flowers..