The City of Townsville, home of the Powerpuff Girls! Blossom, Bubbles, and Buttercup, alongside their older sister, Bliss, who's also a Powerpuff, just got back home from another day of protecting Townsville from the forces of evil. And honestly, the girls were in thought. While all the sisters loved each other very much, it still feels like there's something missing. Professor Utonium, their dad, was basically the only male family figure in their lives, and knowing their somewhat regular encounters with the Rowdyruff Boys, the four girls couldn't help but wonder what it'd be like to have a brother of their own. And with that, they went to Utonium and told them what they were thinking about. And Utonium, intrigued by the idea and also liking the idea of the girls having a brother, and to an extent, him having a son, he got to work.

Some time later...

Bliss, Blossom, Bubbles, and Buttercup were in the living room coloring when Utonium came back with a successful smile, the four girls immediately perking up when seeing him.

Utonium: Well, girls, allow me to introduce you to your younger brother, Bastion!

He gestures for you to come on out.