Flea market day at Ayumu's school, with Orito challenging Haruna to see who makes the biggest profit. Haruna eventually wins when the other busty girls help her out, as well as offer Orito Seraphim's blouse/negligee--at the cost of all his profit. (LOL @ Orito selling H-doujins, though.
Later, Ayumu accidentally drinks an invisibility potion and Haruna is initially not pleased...until she finds out that a Philosopher's Stone of hers is missing. Ayumu immediately makes use of his condition the next day in order to find Haruna's Philosopher's Stone. And he searches among the girls during their physical examination day
... But after getting into a few perverted situations without anyone seeing, Sarasvati manages to spot Ayumu (despite his invisibility--must be her vampire ninja abilities) and even gets a good butt-grope in.
Ayumu manages to explain the situation to Sarasvati and she offers to help out.
And then Ayumu's invisibility starts to wear off...but he's lucky to have an understanding girl like Taeko. Ayumu finds out that Taeko has Haruna's Philosopher's Stone and retrieves it from her. Turns out that it wasn't necessary, as Taeko had the fake Philosopher's Stone (sold to her by Haruna the previous day) and the real one was at Ayumu's house all along. Even with that matter resolved, Ayumu later drinks a potion that gives him X-Ray vision...LOL, the hijinks just pile up. (Though it's still up in the air whether they can recover Haruna's mana from Chris.)