The sisters and Leo learn that there is a Warlock running amok who wishes to steal the powers of the powerful and they know that they must do something to stop him. Things became more than a little amusing at this point when Whitelighter, Natalie, who has known Leo for a long time, turns up on the scene, makes Piper extremely jealous (not sure why) and annoys the heck out of the other sisters.
Natalie is like someone newly graduated from the Marines, with a penchant for crossing all the 'T's and dotting all the I's. She is also aggravating in the extreme in her condescending manner towards the girls, whilst happily carrying on a weird 'click clack' type conversation with Leo.
For me, the best part of the episode was when a very vulnerable Phoebe decided to come clean to Leo about Cole as Phoebe never really does have anyone she can talk to. It was sweet. Complicated, but sweet.