Charmed Again (1)

While struggling with the grief of losing Prue, Piper and Phoebe mourn over the loss of the power of three. However, when Piper tries to contact Prue using the Book of Shadows, she and Phoebe discover that they have a half-sister named Paige. It doesn't take long for the Source to learn of Paige as well, and tries to kill her before restoring the Power of Three.