There's quite a bit going on in this episode. First of all, the naughty demons are taking advantage of the fact that The Source seems to be tempoarily on the missing list and some a trying to build a rival faction. Bad idea. He won't like that one little bit.
Meanwhile, lovely Muses are disappearing and being held captive, which means that many a great speech is not being written and there is a lot of writer's block around. It's up to the Halliwells to try and find out where the Muses are being kept so that they can free them, but that isn't as easy as it appears. Meanwhile, what is going to happen when The Source finds out that his own people are plotting against him? Quite a bit, I imagine! He certainly won't be pleased.