The sisters are on edge. Is it the dreaded synchronization of menstrual cycles, or is it the magical Blue Moon?
Phoebe is worried that Leslie is pulling way from her. This may be because he's literally moving away - his job is done. Plus, PMS makes her weepy, and she's having weird, violent dreams.
PMS turns Paige snarky, mean and jumpy - and makes her involuntarily orb when startled.
Piper wants her family back together, dammit. She's trying to get Leo cleared of all suspicion so he can move back home. And PMS makes her grumpy. Well, grumpier. It also makes her powers blow up electronic appliances.
The Elders want to give the sisters a new Whitelighter while Leo is under suspicion. Piper warns Leo that this is not the right time (and accidentally executes a toaster for emphasis.) Leo tries to warn the new Whitelighter about the sisters' moods, but the New Guy dismisses him. When Leo leaves, New Guy is attacked by werewolfy monsters. Since Leo was the last person to see New Guy, the Elders blame him. At the same time, Paige and Phoebe are arguing that maybe Leo isn't safe to be around - hearing voices and killing Elders just ain't good. Piper dismisses their concerns. Piper resolves to figure out what attacked New Guy, and sends Paige off to check on Brody. Paige stakes out his apartment, and orbs in when he leaves. Brody had spotted her and rushes back in. Startled, she accidentally orbs in front of him. Brody volunteers a blood sample to prove he's not a demon, then warns her that a new power is rising.
Piper resolves to set a trap for whatever is attacking Whitelighters, then sends Leo to Magic School so he has an alibi. The floaty scary head tells Leo it's all for naught. The sisters get a sacrificial Whitelighter to bait the trap (he's none too happy about it.) When the Blue Moon rises, it transforms the sisters themselves into the monsters. They attack Sacrificial Whitelighter and maul him badly, but he manages to get them trapped in a mystical crystal cage. The next morning, the sisters are appalled to see what they've done - but not appalled enough to tell the Elders. Head Elder apologizes to Leo for doubting him.
Paige visits Brody to ask about this new power - what does he know? Not much, just that it's ancient, dangerous, power-hungry - and it killed his parents when he was five. Said mysterious beings are seeking help of their own - the scary floaty heads kill a bunch of demons, then grow bodies to talk to their surviving leader. The heads convince the demon to possess Leo, then attack both the sisters and the Elders. When the possessed Leo goes to Magic School, the sisters follow. They manage to drive the demon out of the Leo, but the Elders miss that pertinent fact. The Elders do notice the sisters turn into monsters and attack them.
The Elders send Leo away and want to confine the sisters to Magic School, but the sisters have figured out it's just the Blue Moon turning them wonky. Since there won't be another Blue Moon for 50 years, they're no real danger. Piper refuses to help figure out this new power unless they get Leo back. They track down the possessing demon, and see a scary floaty head destroy him. They realize Leo wasn't seeing things after all, but is the target of the new power's attack.
Paige tells Brody that the new power was after Leo, and burbles about demons and possessions. Brody doesn't quite know how to handle it. Phoebe thanks Leslie for convincing her to trust in love again, and sends him on his way. Leo tells Piper he can't move back into the Manor until he knows why the new power wants him.