Människan har lyckats klyva atomer och tagit klivet in i en helt ny era. Filmens historia börjar i New Mexico – en liten flicka upphittas i chocktillstånd – en butik är helt förstörd och en kropp upptäcks full med konstig syra. Snart terroriseras staden av muterade jättemyror.

A group of scientists travel to a remote island to study the effects of nuclear weapons tests, only to get stranded when their airplane mysteriously explodes. The team soon discovers that the tests have given rise to crabs mutated into intelligent, impervious, telepathic giants intent on increasing their numbers by breeding, then travelling to populated areas to feed, and which do not intend to be stopped by their discoverers.

Near a small English village, a scientific team is conducting experiments with magnetic fields, the results of which may have military applications but the intensification of which seem to be connected to UFO reports, a series of murders, an enormous insect egg, and a strange visitor with exceptional scientific knowledge.