Ep 21. Look Who's Barking

When a spell cast on Prue turns her into a dog, Piper must find a way to track down a Banshee who is killing innocents, and before long, the Banshee succeeds in transforming Phoebe into one of her kind. If Phoebe successfully kills her first innocent, she will remain a Banshee forever, and Piper must now find a way to solve the problem on her own, otherwise she will lose her sisters forever.