This minor quibble aside, this is a good, quite complex story with lots of characters. The locusts, led by their magician-queen, are looking for the magic lake "Lok-Nor" which moves around of its own accord, seemingly at random. Wherever the locusts go (in their "tens of millions" as the Narrator puts it) they devastate human communities by causing famine and death.
The plot keeps moving throughout and it would take more effort than I can be bothered with to summarise it, so I will just mention some of the highlights:
Pigsy is afraid of the locusts and faints, and Monkey puts him in a "natural ice cavern" (complete with refrigerator-style door) to recover.
Sandy's first encounter with the locust queen results in her magic giving him a pig's snout in place of his nose, upsetting him greatly while amusing Monkey. Monkey does eventually reverse the spell.
Monkey is unable to defeat the locust queen at their first meeting, due to her magic powers, but he does eventually save her from her generals when they attempt to murder her.
Eventually Sandy and Monkey combine their magic powers to create a new lake for the locusts to live by. Presumably, while it may be magic, this one doesn't move about.