A group of scientists & the like are assembled to investigate the disappearance of another group like them who were sent to an island to study the effects of radioactive fallout from one of the first big H-bomb tests. Unfortunately for the crew, one of the effects was creating a giant crab monster (not a spoiler, it's basically the name of the film.) But one of the real mysteries is what happens to those who are attacked by the monster, which by the way is pretty well done in comparison to other early Corman creatures.
Fair acting, Pamela Duncan is lovely but seems to have to cook for the others due to being the only female & despite being a doctor. And yes, that's Russell Johnson, Gilligan's Island's Professor who predictably expresses interest in the girl & spends most of his time being unable to repair the radio to issue a distress signal. Talk about being typecast.