Mamoru goes bad, and Ranma ends up with Usagi.

[Spokavriel]

Do you remember how many times I would mention a section in either Heir to the Empire or some other one that visits vaults on the moon confusing it for that fic? I can never remember to place the events of that fic with its actual title.

[Lioconvoy]

...really it is that fic? I don't recall Ryoga being related to Mamoru at all, that's why I didn't think it was it.

[Drawde2]

Ryoga's relation to Mamoru isn't a big thing in that fic. It's mainly an excuse to introduce the characters to each other. It quickly gets overshadowed by other things in the fic, especially the truth about Endymion.

[Lioconvoy]

Which is probably why I forgot about it and dismissed the story when I saw that they were related.

[Wyrd]

I thought at first that you were thinking of Fist of the Moon, but later clarifications of the details of the story disqualified it.