Mizusawa's been stalking and collecting began way before Nishimori even met Mika in that alley. Noda was taskedblackmailed into both protecting "the shack" and "Mika's virginity" so that Mizusawa could eventually claim it, after trying to back out of their initial "partnership". This is also where Noda heard the line "I would die to have your first time" from Mizusawa.
We then get treated to a cliffhanger, as usual, of a hand (presumably Mizusawa's) about to burn the whole shed down to get rid of the evidence (presumably tipped off by a hospital staff where Noda was because of his contact with Nishimori) shack.