From MangaHelpers:

Rinosuke's useless and vagrant father is now dead. Now, at the age of 16, he is completely alone and will have to quit school to survive. On the night his father dies, a naked woman with large breasts falls from the sky and lands on top of him. She introduces herself as Sakamoto Marina, the future wife of the future Rinosuke. She states that she used a time machine to come back to the past, and start a new life with the younger Rinosuke. Rinosuke's new life with his future(?) wife begins.[DrCoke]