No God in Eden Vol. 3 (manga)

After their seven-day ordeal in the storage room, Takai and Nishio have returned to their normal lives as best they can and started dating in secret. But their experience during Nishio’s heat has irrevocably altered them, with each boy noticing a new level of possessiveness over the other. With the arrival of spring and their third year of high school, a second heat threatens to arrive, and a silent observer has begun to take notice of just how much Takai and Nishio have changed.

Will this alpha-omega couple be able to keep their secret hidden? Or is there an intruder on their private world?