For years, 20 million shape-changing Zygons have lived among us in secret. They wear human form, hiding in plain sight. Now a fanatical Zygon splinter group seek to expose their own kind and provoke a conflict that will force both sides to the brink of Armageddon to ensure their own survival.

It took three Doctors to broker a fragile peace between Zygons and Humans. Now the 12th must face the fallout alone. With his allies compromised and his companion believed dead, can he stop the world from plunging into war?