The Doctor arrives in an underwater base on the planet Flydon Maxima — known locally as "Despair" because hope is lost for all life there. The scientific base has for centuries been monitoring the results of planetary warming — the slow melting of the glaciers at the south pole and subsequent rise in sea levels. Curiously their most advanced monitoring equipment was created and donated by Varlos. The Doctor is helped by a little girl called Gisella. Why did Varlos risk coming here? What has caused the ice to crumble so rapidly now? Can the Doctor evade the rampaging Dreadbringers, and sort out the attacking sea monsters? Who is Gisella and will she and the Doctor manage to escape in the TARDIS?