basically Cartman would be hypnotized by Wendy and almost get eaten by Stan and Wendy. But then Kyle turns into his Mercy form and attacks them – but since they’re two against one they can kind of hold him off. 

Then Cartman suddenly gets swallowed up by the ocean, and we meet Sheila, who found the strength to break out of her tomb when she felt Kyle’s Mercy form. Kyle pries Cartman out of Sheila’s grasp, and just as he’s about to drown, Kyle figures out how to make Cartman breathe underwater. After that he realizes he has magic in this form. So after a heartfelt reunion after calming his mother, he and Cartman explore the ocean floor together, still in his Mercy form.

And you’ve seen enough hentai to know how that went.

But anyway after all that Cartman finally admits that he’s worried about Butters, and they go back on land to rescue him from Clyde. It’s pretty easy to overpower them when Kyle’s in his Mercy form. 

Then after that, I hadn’t figured out which ending to choose, but it was either that Eric turned into a merman and lived with Kyle underwater, or Kyle lived with Eric as a human.

But I was much more interested in M4s plot, and still am, so I’m gonna go with that.