Who are the invaders? An undersea quake uncovers mysterious metallic capsules, which have been hidden for millions of years. In one capsule brought aboard Seaview is the strange and powerful Zar, who claims his people lived on earth eons ago. He wants the rest of the capsules retrieved and all his people revived. Nelson and Crane must decide whether Zar and his technologically sophisticated people can co-exist with humans. And Zar's blood contains a deadly virus, normal for his people, but lethal to humans. The slightest injury to him could spell doom for the human race!