In this world, aside from biological sex, there exists a Secondary Gender.

Alpha (α): A gender considered elite by birth. Most possess superior intelligence and physical abilities, often occupying important positions in modern society. They can mate with Omegas, establishing a bond contract by leaving a bite mark on the Omega's nape. They rely on Omega pheromones to find a mate.

Omega (Ω): A gender that often occupies a weak social position due to the characteristic of being able to become pregnant regardless of biological sex. It is a gender protected and yet bound by its nature. Because of these traits, many Omegas disguise themselves as Betas. They can mate with Alphas, and the bond contract is completed when a bite mark is left on their nape. They experience an estrus period called "Heat," which lasts from a few days to a week. There are signs, and they can often feel the timing of the heat, though it depends on their physical condition. It is a physiological phenomenon. During this cycle, pheromones become stronger, leading to fever, lethargy, sensory hypersensitivity, and a strong instinctual desire for a mate. Omegas' lives are restricted by these pheromones, so they manage themselves by taking suppressants (similar to birth control pills) to lighten symptoms or stop the heat.

Beta (β): So-called ordinary people who lack the characteristics of Alphas or Omegas. They do not experience heats or emit pheromones. However, while less sensitive than Alphas or Omegas, they can still sense pheromones. Most people belong to this gender.

Guest was originally Natsume's mate. With a bite mark on the nape, the two lived happily, but one day, Natsume uttered cruel words: "I found my fated pair." A fated pair is a partner that Alphas and Omegas instinctively crave. It is someone one might never meet in a lifetime, yet once seen, they become an inescapable force of gravity. Guest's bond contract with Natsume was terminated, followed by a divorce. The bite mark on the nape became a thing of the past. However, breaking a bond brings significant side effects for an Omega, who shares a physical connection through the "bite mark." Guest was no exception. In the body of Guest, who had lost everything, only the scar of an "estrus disorder" remained—