Nagi is a diver from a quiet fishing village where a series of strange incidents have been occurring recently. One day, a stranger takes her to an office where she is asked to recover sunken treasure at Kaijin Reef. Nagi agrees to do so, but during her dive, she gets exposed to strange radiation. Afterwards, she starts to experience spells where she exponentially grows in size. This causes a lot of problems for her and the people surrounding her.
My useless brain read the title as "Giant Worm", and then I saw the cover and thought "okay, that's clearly the octopus back there, so is she supposed to be a worm? That has got to be the laziest, most fan servicy bullshit worm costume I've ever seen"
Then I read the summary hoping she wasn't the worm, and then was confused about why it didn't mention a worm at all! I figured it out, eventually...