As odd a thing as it is to say, perhaps we should begin where the previous tale stopped.
The evil Bowser Koopa and his children, the Koopalings, were after a magic lamp that contained three genies (or rather, ghost-genie-hybrids, as they called themselves) who would grant a wish each. To obtain the genies, Bowser had to pass through an opening that would only accept a pure soul, which the Koopas were obviously not.
The pure soul turned out to be Double D, who was captured and forced to find the lamp. After a few accidents (caused by Double D's companion, Ed) Double D found himself with the lamp possessing the ghosts. He used his first wish to woo Nazz, a girl he liked. The second wish was used to save him from the Koopas, who tried to kill him and get the lamp themselves.
Eventually, however, Bowser did manage to obtain the lamp and wished himself a magical ruler of the Mushroom Kingdom and blasted Double D to the far regions of Iced Land. Double D returned, and with the help of his friends, defeated Bowser by tricking him into wishing himself into a genie. By doing this, the Koopa found that he was bound to his own lamp, which Double D used to wish everything back to normal, that no one but a few friends would remember this, and that Bowser was trapped in the lamp forever.
While one of Bowser's sons, Jr, wanted to wish him free, the other Koopalings, who felt overworked, decided to take a break from evil and stuffed Jr into the lamp alongside Bowser and threw it into a stream.
The stream turned into a river, the river turned into an ocean, and eventually, somehow, the lamp washed up in a desert where it sat for a few weeks.
Back at camp, the kids practically forgot about the lamp and Bowser. Aside from one or two minor run-ins, the Koopalings never showed up around camp. Yes, things were much more peaceful now. But Bowser and Jr were still out there in the desert...waiting...and getting pretty bored in the process!
Actually, I had the feeling I'd eventually write this story. For that reason, aside from a small cameo, the Koopas have been taking a break for a while. It was partly a "can the series work without them?" experiment. If I didn't do the sequel, I'd just stick Bowser back in the series eventually with no explanation of how he returned, but thankfully, his reasons will now be revealed.