An absolutely brilliant episode, P3 H20 works because of the new setting they use for the episode. In various TV shows, taking the characters out of their familiar settings only occasionally works but luckily, Charmed does so with perfection.
P3 H20 has the sisters investigating a series of drownings at Camp Skylark, the camp where they stayed as children, and discover that the water demon responsible for the drownings is the same demon who drowned their mom Patty. They also meet the former whitelighter who had an affair with their mother.
This has to be one of the best episodes of the season with some extremely dark scenes (the drownings in particular) and Holly, Alyssa and mostly Shannen give brilliant performances. Shannen is very convincing as Prue realises that she's the one who has to kill the water demon and it's neat how history basically replays itself, first with Piper freezing Leo just as they're about to kill the demon, the same thing that Patty did to her whitelighter Sam, and also when Piper and Leo realize that they weren't the first whitelighter-charge couple. Brilliant.