Based on the erotic game by Orcsoft Team Goblin. 
 
Naoki overheard his mom having an embarrassing conversation about him with his next door neighbor Shiori. Yet even so, he's still looking forward to the next five days. Naoki's close with his mom, but since they live together he sometimes wants to stretch his wings and fly a little crazy. He wants to be able to fap, without being concerned about his mom or anything! 
 
One time Naoki ends up attending an event for his mom's work, but the whole time he was thinking about that super hot porn video his friend showed him. And Shiori's there. They've been neighbors for 10 years, and it wouldn't be strange to call her his second mom. Well, except for the fact that he found her body incredibly sexy... He wouldn't mind losing his virginity to her. 
 
But just when Naoki had decided that was impossible, she starts to comment on how handsome he's gotten... 
 
(Source: DLsite English, edited)

Kaa-chan no Tomodachi ni Shikotteru Tokoro Mirareta. THE ANIMATION