Season 1: Episode 3: The Son

"'What is this turn based system?!'"

Synopsis
In this episode, Bowser Jr, Mario, and Luigi have a showdown in a place called: The Nightmare Realm. In this battle, Bowser Jr is stronger than ever for some reason. Mario and Luigi use their basic attacks to defeat Junior, while he fights back. Mario and Luigi make it with only a sliver of strength left. In the second phase, Bowser Jr gets help from two friends.

Fortunately, Mario and Luigi defeat them all and Bowser Jr escapes using a time machine, never to be seen again. It's later revealed that Jr dies in a microwave nearby.

The next episode happens during this one.