Pigsy goes to rescue a girl from a Slug monster, but it turns out she doesn't need or want rescued, and in fact the monster is glad to be rid of her since he already has a wife and three children (also Slug monsters).
Tripitaka eventually saves the day by accident when he attracts the girl's interest away from the Slug and to himself, and he has to flee from her attentions upon the horse.
There's not much more to the plot than that, and most of the interest comes from shapechanging shenanigans - Pigsy disguises himself as the Slug to woo the girl, and this leads to further mistaken identities regarding who is really whom.
There's also the question of why the girl has so much salt to hand with which she can threaten her Slug-husband into obeying her - why so much? Where does it all come from?
The answer, of course, is that it's just a silly story and not to be taken seriously. Fair enough if you like that sort of thing, but for me it was just too silly for my tastes.