In this world there is a form of human that is kept as pets. Basically, they are called mutts, and are physically humans with the brains and minds of dogs. It's passed genetically, with the trait of ending up like an animal as a trait born is almost anyone who is a child of a mutt. While they are generally wild, humans have invented collars that track the gene in mutts and tether them to the will of human owners, so that they remain obedient and eager to please them. Mutt girls tend to go into heat whenever they ovulate. In such a state they care very little for things like gender or consent, making owning one mutt challenge and two mutts a trial.
The story would follow two friends. One is a guy who's owned a mutt girl all his life. It's been about two years since he's come to terms with his responsibility to control her heat, and when he moves out of the house, takes her with him. He's friends with a girl who admits it to few people but is one of the 1 in 1000 people who were born human despite being the child of a mutt. She's generally bubbly and friendly, but her parentage is her deepest secret that she is least edjustied to. Curious about her herritage and likely at some level turned on by the idea of being a mutt, she privatly asks him if she could try on one of his mutt's spare collars. It's proven that mutt collars have a slight effect on some of the rare few who were born human with varying degrees based on genetics. When it's on they find that in connecting to her latent gene, the collar actually lets the gene become dominant, making her little more then a mutt with a slightly smarter intelligence when she puts it on. When she takes it off and reverts back to normal the two are shocked, her more so because of just how much she enjoyed becoming a happy pet. A week later she shyly asks to try it again, mentioning that she knows she'll be in heat if she were to try it on that day. So begins the story of how he trains his friend as an animal whenever they agree, but at what point does it go to far? Will she learn to control her needs, or will she choose to keep a collar on permanently?