So now we know why Arisa had that "mysteriously horny feeling" after seeing Mari cry in like chapter 7.
My prediction: We get the flashback arc, chronicling the development of their original relationship and representing the return of Arisa's memory. Then she announces to Mari that her memories have returned, and promptly begins a full-on make-out in front of everybody they know. The embarassment is so critically massive that Mari loses her memory, and we enter phase 3, where Arisa has to win back the heart of her amnesiac wife.