In the present, Piper and Leo are in front of the manor saying goodbye to Phoebe, who is going to finish the last week of her book tour. Phoebe notices Piper is glowing and asks if she is pregnant again. Piper says she is not pregnant, but that she and Leo have just been perfectly happy lately. She admits that she finally feels like she can relax. The sisters then comment about his butt, at which point Leo asks what they're talking about. Their conversation is interrupted by the arrival of Montgomery Knox.
Inside the manor, Knox reveals that when Leo and Cole came to him for information about the Ancient Athame, Knox and Leo made a soul agreement. Knox handed the information over in exchange for a favor from the Charmed Ones, and if Leo does not fulfill his end of the bargain, he will die. Knox comments on Cole's death and tries to give his sympathies to Phoebe, though she becomes upset and leaves. Knox notes that the deal included all sisters, though Piper replies that she, Paige and Leo will be enough.
As Phoebe leaves, Paige arrives and Knox begins telling them about his past. He reveals that he is the child of a witch and a warlock. As a young man, he made a soul agreement with a sorcerer named Aldar Kose. The sorcerer would train him in exchange for a favor years later. After his training was finished, Aldar disappeared and Knox opened Knox Academy.
Last month, Aldar appeared at the academy and revealed he was planning a ritual known as the Advent, the resurrection of the Old Ones. He had managed to manifest the souls of the Old Ones in the form of seeds, which could hatch in the bodies of powerful witches. He planned to use the students at the academy as vessels. Knox wants to save his students, which is why he made the soul agreement with Leo. Piper wants to vanquish Aldar, though Knox tells her that because of the soul agreements, Knox and Leo would die as well.
Piper orders Leo to contact Prue for help and sends Paige to the academy, while she confronts Aldar Kose. Phoebe and Paige call their husbands. Meanwhile, Prue refuses to help her sisters because she feels abandoned.
Aldar Kose clones himself and teleports to Knox Academy to begin the Advent. Paige sees him arrive and orbs Piper and Leo to the academy as well. When they arrive, the students are fleeing. Knox arrives and they all begin fighting the clones, though they cannot figure out which one is real. Phoebe then arrives and finds the real Aldar with her empathy.
Aldar fires the seeds of the Old Ones at the students before Piper can freeze him. Knox jumps in the way and is hit instead. His eyes glow and he says it's a beautiful symphony before collapsing. Because he kept his end of the bargain, Leo's life is spared as Knox dies. Piper then vanquishes Aldar Kose.
At the manor, Piper and Leo lie in bed and discuss what happened. Piper admits that after years of resisting her magic, she is finally able to accept it as long as she has him. Leo then tells her there is one more thing they need to discuss, Prue.
At the academy, Knox rises from his grave, possessed by the Old Ones.