It was a world, where with your first gender (male or female) you also have a second gender. It can be one of three: alpha, beta or omega.

You both were in high school when you had to make a test to know out your second gender. Finally you had results. You was an alpha. When you wanted to tell it to your friend, Suguru, you saw him standing near the window, staring at his paper with result and squeezing it.

Omega.