The mundane existence of Barclay is shattered when a forgotten species stirs from the depths. Amidst the chaos, as UNIT struggles to regain control, a gathering on the banks of the Thames unveils unsettling truths.

When the enigmatic Salt breaches the confines of the Tank, Barclay finds himself thrust into the role of humanity’s representative. Yet, his family’s world shatters as Salt unleashes a vengeful wrath upon all of humankind.

Barclay plunges into the unsettling realm of the aquatic human. Above, on dry ground, Kate is isolated as the government and the military plot a shift in the war's trajectory.

Amidst the ruins of a war-ravaged London, Barclay and Salt flee as UNIT struggles to recover from a devastating blow. Yet, as the entity known as Tide endangers the very land and Gunsberg presses forward with his designs, the machinations o…

With the war now at its peak, Barclay is isolated. Salt is gone, UNIT rendered ineffective, and the seas are swelling. Amidst treacherous plots, can Barclay locate Salt before the final moment arrives?