Thank god, that's a weight off our shoulders.
What was this, out of curiosity? Haven't read PMKM.
It was a confluence of two events that occur right around the beginning of the manga: Hijiri field testing
Evil Nuts Asunaro Almonds and the Pleiades finishing Kazumi #13.
The aforementioned Almonds are basically hypodermic needles of Grief; when put into a human, they bring out the darkness in one's heart to the fore, causing them to get themselves tied up in some nasty shit while slowly transforming them into a weak pseudo-Witch (unknown if they have any effect on males, as all of the victims in PMKM are women; when used on a magical girl, it attempts to Witch them out, and when used on Kazumi, it degenerates her chimaera body, causing her to briefly go berserk when sufficiently stressed).
In this particular case, the victim is a female detective from the local police department-- she's (one of) the only woman in the force, and thus the subject of scorn and ridicule; with the influence of Hijiri's weapon, she cooks up a false flag bomb plot that she can bust to inflate her own prestige: A local restaurant owner's being forced out of business by the newly-opened mega-mart nearby, so she hooks him up with a big ol' briefcase full of bomb. However, he ends up colliding with the girl transporting the new Kazumi clone, who happens to have had an identical brand of giant suitcase; they get mixed up, and when he tries blowing up the shopping center, he gets the 'is this real life' experience of a nude amnesiac middle school girl bursting out of the luggage instead.
Needless to say, he needs a day to rethink his life decisions up to this point. They end up heading back to his place-- I don't remember offhand if Kazumi succeeds in persuading him not to go through with the bomb plot, but the Pleiades ring him up to organize an exchange, since they discovered they got a bomb where a Kazumi was supposed to be. Right as they're set to make the exchange, though, the bust happens, and the bomb gets armed accidentally; Umika and Kaoru make a timely intervention, but since nobody present knows how to disarm a bomb in thirty seconds, Kaoru kicks it into the ceiling instead, where it's revealed the bomb was a fake, exploding into confetti. Police girl is publicly humiliated, Kaoru and Umika take Kazumi home, take her shopping the next day while they catch up, leave her home that evening while they're out hunting, police girl swings by to interrogate/murder Kazumi as the Evil Nut finishes transforming her. Fortunately, its possible to beat her up until the Evil Nut runs out of power/is destroyed without killing the host, as Kazumi discovers she's (technically not exactly) a magical girl.