Mmm, you were the one who explained it to me, right? I don't remember exactly, but I'll try and let you know if I remember how.
I found it. You have to go to Annie's octopus level, and while there, you have to call Rose several times until she says she's tired of you calling her repeatedly. Then you go to where the succubus that appears when you break the red crystal is, talk to her and go up to the surface, you have to talk to Mason and the NPCs that are up there too. Then you go back to the beginning and talk to Rose and ask her to call Melanie. And that's it, you can now return to the surface where Mason was alone and should be with Melanie. That's the animation.