The demon-filled dimension known as "Mato" has opened to threaten humanity, and a type of "peach" tree that grows there yields fruit that confers special powers to those who eat it, giving humanity a fighting chance, but this only works for women. As such, as protectors of humanity, women rise in social status while men are relegated to second-class citizens scraping for basic occupation and acknowledgment.

Cue Yuuki Wakura, a domestically crafty and laborious high school senior whose older sister was taken during a Mato event 5 years before; while lamenting the hardships of the life ahead of him, an event opens before him, and while being rescued from demons by Captain Kyouka of the Anti-Demon Corps, she confers his powers as her "slave" and turns him into a mighty beast mount that mows down the demons.

However, there is a surprising price to pay for being her "slave"... and it's not him who has to pay it.

Watch Mato Seihei no Slave Online