A gigantic, mutant crab is sighted terrorizing various islands in the South Pacific. When the American military learn of this, they assign Commander Hartnell to put a stop to the crustacean threat that has become known as Titano. Hartnell's crew has several encounters with Titano and they eventually lure him to the Arctic. Hartnell maneuvers his ship so deftly that he manages to trap Titano between two giant glaciers.; A derelict claims to be from another world and says that another planet will collide with Earth within days, but he is believed mad. He escapes and decides that he really is insane, hiding in a cave while, unknown to him, astronomers observe a nearby planetary collision and realise that he really was an alien.; A space explorer cannot rid his ship of vicious alien spiders, and is forced to sacrifice himself to save Earth.; A scientist selfishly uses a youth serum to gain an advantage over other people, but the serum ages him backwards into non-existance.