Makoto's parents were killed by a monster, but she was saved by a magical girl. In order to become a magical girl, Makoto asks Shouya, her childhood friend and closest person to her, to be her partner. They have feelings for one another. They don't know why partners are necessary, though.
After they arrive at magical girl headquarters, Netorastia (lol), a monster attack happens. The couple learn why partners are needed after their bond is formed from Kyuui, the Kyubey-like creature. Magical girls get their energy from the disparity in emotional states between partners. The magical girl receives magical energy when she's feeling good AND her partner is feeling bad. The most powerful method seems to be when the girl is feeling pleasure and the guy is in despair. So, Kyuui has Makoto fuck another guy in front of Shouya to get magical energy. Initially, they refuse, but after some existential goading from Kyuui - Makoto wants to get revenge for her parents - Makoto decides to go through with it against Shouya's wishes. During the act, Makoto goes form unwilling to complete whore during the sex, which of course places Shouya in despair.
After the act, Makoto returns to her senses. Kyuui explains that when magical girls lack magical energy, they receive more pleasure when fucking someone other than their partner. However, once their magical energy is restored, they return to their normal state. After Makoto transforms, Kyuui asks if Shouya hates Makoto after this, Shouya answers that he still loves her. Kyuui states that this is proof that the bond is in effect.
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Interesting idea! This guy almost never writes sequels, but I'd like to see how far this goes. Kyuui says he doesn't like meaningless actions: when he forces Shouya to watch Makoto having sex, he says the sex would be meaningless if Shouya can't watch. There are a few possible avenues that could come from that. For example, if Shouya were to somehow no longer love Makoto, she would probably lose her powers. BUT, if the contract forces Shouya to love Makoto despite everything, what's to stop him from, say, killing himself for freedom? I doubt any of that is going to be explored, of course.