Smith lets drop some news: the interspecies exchange act may soon be revised to allow marriage. They just need a good test case.
The girls are sure they'd love to be a test case. But only one of them can be.
When a full moon sets their animal instincts loose, shenanigans ensue.
A slime girl appears. They name her Suu.
It's not clear how much mentality she actually has at first. She mostly seems to mimic others.
But when one of the neighborhood children is nearly hit by a car, she shows her own sort of heroism.