Main story: Aoi's (Omega) family past away and he was taken in by Jirou's (Alpha) family as a servant. He became the servant taking care of Jirou and his heat started. Just as he was about to leave the house due to his heat, Jirou stopped him and they bonded.
Side story: Miyauchi (beta) and Kudou (Alpha) are servants of Aoi and Jirou respectively. As they interact with each other more, they slowly get into a relationship full of many other factors to consider. (They got married)
Kashikomarimashita, Destiny