In order to save the sacrificed people [well, actually just for the professor], the savior traveled through time and space and went back to the time when he was eleven years old in the hope of a perfect pass. According to Professor Snape, "Mr. Potter, who thinks he can handle everything, is getting more and more bold."
Unfortunately, the beautiful plan went a little wrong. Harry met a little boy who was extremely happy about the unexpected time travel and wanted to hide his identity and witness the whole process of his father's love.
... That boy was his and Severus' son in another world.
[The HP/SS of this article is correct, and may involve other CP~]