Part 1: Childbirth.

Toshiro is Naruto's real father. His mother died after Naruto's birth. Toshiro only spent 2 years with Naruto before he was ordered to return to the Seireitei, well he was forced to return. Every single day, Toshiro felt guilty. He wished he could've taken Naruto with him but he couldn't since Naruto is a human but has Shinigami powers within him. 10 years later, Toshiro, Ichigo, Rukia, Jushiro, Shunsui, Rangiku, Nanao and Renji are ordered to go to the Shinobi World and protect the people from hollows. Toshiro is glad to return and meet his son but was shocked on how his son was treated because he was not normal. Toshiro was also mad 'cause someone made Naruto into a Jinchuuriki. In this story, Toshiro is always in his adult form.