Donna and the Doctor take a holiday – in the beautiful underwater city of Vallarasee. Things have changed since the Doctor last swam through it: Vallarasee is now enclosed inside an airdome. Judoon patrol historic sites, and instead of tourists wearing helmets to breathe, the native Fins are forced to adapt.

But leaks have begun to trickle into the dome, and the Doctor and Donna must convince the Judoon that disaster is imminent. Or else thousands will be trapped, as the waters rise…