When the legendary pirate queen Zheng Yi Sao, alias "Madam Ching", awakens an antediluvian Sea Devil extremist, the Doctor's TARDIS is pulled off-course and Team TARDIS must try to work together with the historical rogue to save the entire Earth from global flood. But amidst the high-stakes, high-sea adventure, it soon becomes clear the stakes for everyone involved — even the Thirteenth Doctor and Yaz — are much more personal than a legendary sunken treasure, or even the all-important Keystone...