The Elders suspect Leo of killing Zola and plan to assign a new Whitelighter to the Charmed Ones. Leo goes to the new Whitelighter, Danny, who is meditating under a blue moon, and tries, without success, to talk him out of his new assignment. Shortly after, Danny is mauled by three wild beasts. At Magic School, the Elders heal Danny and suspect Leo of the attack.
Phoebe goes to talk to Leslie, who is leaving the paper soon. They are attracted to each other, but he is moving to Los Angeles and then to Philadelphia, and her activities as a witch constantly interfere with their relationship.
Paige goes to check out Agent Brody. He walks out of his apartment and she orbs in; he walks in and Paige, jumpy, briefly orbs. He admits that he has been following the Charmed Ones and knows about them being witches. He warns her of a new power rising, like none ever seen before.
Piper checks the Book of Shadows, regarding the attack on the Whitelighter, but finds nothing. Piper proposes setting a trap for the beasts. A new Whitelighter, Marcus, is in the attic. Phoebe mentions the blue moon and Paige remembers reading about blue moons in the Book of Shadows, something unspecified if there are two blue moons in a year, which happens once every 50 years. As the blue moon rises, the sisters transform into the beasts and attack Marcus. The sisters awaken to find themselves in their own crystal cage. Paige orbs a crystal away and the sisters escape. They realize that they must have attacked Marcus.
Brody tells Paige of some beings that had power, went away, and are trying to come back. These beings killed his parents when he was five years old.
Three floating heads, such as have been taunting Leo, transform into the Avatars. They come to a demon to try to turn Leo away from his family. The demon comes and possesses Leo.
It is now the last night of the blue moon and the sisters prepare to cage themselves. Leo, however, has been behaving strangely and Piper suspects him. He attacks Piper, but she escapes, and he orbs to Magic School to attack the Elders. The sisters throw potions and free Leo, but the Elders are still angry. The sisters transform into the beasts, but the Elders fight them off.
All is cleared the next morning, when the sisters tell a shocked Odin that it was because of the Blue Moon that they became those beasts. The sisters find the demon and try to interrogate him, but one of the floating heads vanquishes him.
Leslie leaves the newspaper and Phoebe returns to work. Leo believes he has become a threat to the family and chooses to stay away, over Piper's objections.
- All Periods Are PMS: In this episode, the three sisters go through PMS. Phoebe becomes overly emotional, Paige gets jumpy and Piper becomes irritable.
- Bittersweet Ending: Leo is fully exonerated, but what happened in the interim makes him decide to stay away from the family.
- The Bus Came Back: The mysterious talking heads from the previous episodes are revealed to be the Avatars, who were last seen two years ago.
- Frame-Up: The Avatars have the demon possess Leo and attack the Elders in order to make him an outcast.
- Jekyll & Hyde: The monsters the Charmed Ones are hunting turn out to be the Charmed Ones themselves, transformed by the blue moon.
- Lunacy: The appearance of two blue moons in one year is what turns the Charmed Ones into demons.
- Oh, Crap!: The general reaction to the sisters' transformations.
- Real After All: The sisters' reaction to seeing the demon heads that Leo's mentioned before.
- Selective Obliviousness: When the Whitelighter is mauled by something clearly not human, Odin immediately accuses Leo. When Leo asks if a Darklighter did this, Odin quickly dismisses it, because it doesn't look like their MO. The fact that it doesn't look like Leo's MO neither doesn't seem to bother him.
- Weird Moon: In real life, "blue moon" merely means 13 full moons in a year, as opposed to 12. In this episode, the moon is literally blue.