Makino is an expressionless corporate spy who gets caught. Instead of firing him, Kamina blackmails him into moving to his house, where he can keep an eye on him. Makino ultimately agrees, seeing it as an opportunity to gather more intelligence directly from a powerful source. They sleep in the same bed, eat together, and go to work together. Within two chapters, Kamina purposefully allows him access to all the information he needs and waits for him to make his move. He chooses to give up on being a spy and claims to have lost this battle of wits. Kamina says he was the one who lost, for he has fallen in love, and all he wants is for Makino to smile.
From Chapter 3 onward, they are happily together, and that's how they remain.
I love the framing of the relationship and how naturally Kamina pursues Makino. It's almost like he didn't even realize that's what he was doing.
"Since when have I become this scared of betraying his trust?"
"I want to be the one who makes him smile genuinely from the heart."
Ou-sama no Koi Asobi