So they're making a pie, duh, and Mamori gets some ingredients on her face. Harley's feeling silly so she licks if off which makes Mamori get all blushy and embarrassed and "NO, HARLEY" in that special "NO, HARLEY" tone of voice. But she liiiiikes it, really, so Harley laughs at her and gives her a little kiss and Mamori gives a little kiss back and oh my what is going on here how interesting and it goes on from there ending with sticky pie sex.