Oku met Nana again after they slept together long ago. In the past, Nana is a The Prince of Good Fortune, there was a jinx that any woman who slept with him would be come lucky and fulfil their wish. So, Oku asked Nana to try sleep with him if he can also make guys lucky as well. However, Oku's purpose wasn't about luck but his secret feeling.

(Source: MU)