Basically, Kintaro was traveling around the countryside on a motorcycle. He (almost literally) runs into the bicycle of the original Golden Boy: Golden Girl. She ends up taking his virginity in the woods. The two become lovers and attend school together.
One day, Golden Girl suddenly disappears. Kintaro waits for her diligently, but he eventually breaks down after his middling years at school and starts seeing a younger student. Kintaro takes her virginity. Their relationship seems to be going well.
Golden Girl suddenly shows up again just as Kintaro was fucking his new girlfriend with their apartment door accidentally left open. He is horrified, but Golden Girl forgives him immediately and without any show of anger. She explains that love is something which she chooses not to covet jealously.
Kintaro ultimately can't decide between the two loves of his life, so he drops out of school after having acquiring all necessary credits for graduation. He becomes the Golden Boy and basically follows life by the life creedo that was Golden Girl's.