Marisol explains the way to save Macy to Maggie, Mel and Harry. The sisters must go back and convince her they will never abandon her, then they must transfer The Source out of her body and into a necklace.
Harry: You are witches. Witches who are destined to save the world from impending doom. That's right, you are the Charmed ones. The most powerful trio of witches. Mel: He's the new women's studies teacher. I knew something was off about him.