A powerful warlock shows up claiming that he made a pact with the sisters' mother in which he spared her life in exchange for the girls' powers. Fleeing him, the three sisters go back in time to the 1970s when their mother was still alive and they try to prevent the pact.

The sisters must face and destroy the demon that was responsible for their mother's death 20 years ago. In the process, they uncover a chapter of their mother's life that they never knew about - one that disturbingly mirrors Piper's current relationship with Leo.

On the day of Piper's marriage to Leo, Prue's Astrally Projected-self adopts an uninhibited, wild personality that threatens to ruin the wedding and place Prue in jail for Murder.

While the sisters must deal with the possibility of being separated as the newlyweds plan to move out on their own, they recall a time before they knew they were the Charmed Ones, when the girls were coming to grips with the death of Grams and Phoebe's departure to New York.

Mourning the death of Prue, Piper and Phoebe must now come to grips with the loss of their oldest sister, and the loss of the Power of Three. However, The Source discovers an unknown sister - the daughter of Patty and her whitelighter Sam, who had been kept hidden, Paige, the now-youngest Halliwell can restore the Power of Three and reunite the sisters' strength as witches.

Mourning the death of Prue, Piper and Phoebe must now come to grips with the loss of their oldest sister, and the loss of the Power of Three. However, The Source discovers an unknown sister - the daughter of Patty and her whitelighter Sam, who had been kept hidden, Paige, the now-youngest Halliwell can restore the Power of Three and reunite the sisters' strength as witches.

Piper, Paige and Leo struggle to convince Phoebe, now a mermaid, to come back to her life as a Charmed One, when she decides to ""swim"" away from her problems with Cole. Meanwhile, Piper casts a spell to stop her panic attacks but instead of facing her fear, she suppresses it, endangering herself and her baby, when a sea Demon comes after her mermaid sister, Phoebe. In order to help Phoebe, Paige takes drastic measures and enlists the aid of Cole, and his newfound demonic powers.

When Piper and Leo begin to have marital problems, Piper casts a spell to allow them to literally see their past together. However, the spell goes awry when Phoebe and Paige are sent back in time to relive Piper's memories, and a warlock bent on destroying the Charmed Ones tags along for the ride.

When Paige and Phoebe summon Grams for Chris's wiccaning, Grams puts a spell on the children to try to end their sibling rivalry, but instead it reverts the sisters back to bickering teenagers. Meanwhile Phoebe, still acting like a teenager, attends an award ceremony with Les, and Leo consults a demonic seer to find out who is behind the attack on Wyatt.

Piper & Leo, with help from Coop's ring, must travel back in time to change the events that lead to the deaths of Piper's sisters. Meanwhile, Billie uses her power of Projection to do the same thing; many familiar faces gather to help Piper in her time of need.