Barclay becomes humanity's ambassador when the mysterious Salt emerges from the Tank. But his family's life is torn apart when Salt wreaks revenge upon all mankind.

Barclay makes a terrifying descent into the world of Homo Aqua. Back on the land, Kate stands alone as Downing Street conspires with the army to change the course of the war.

Barclay and Salt go on the run in a war-torn London as UNIT reels from disaster. But when Tide threatens the land and Gunsberg advances his plans, the Severance schemes demand blood.

Barclay stands alone, Salt is lost, UNIT is powerless, and the oceans rise as the war reaches its climax. While traitors conspire, can Barclay find Salt before it's too late?