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.

Paige takes the Book of Shadows into her work to cast spells which help her co-workers. Meanwhile, Piper, who is filled with anger over the death of Prue and the recent actions of Paige, is possessed by a member of a supernatural band of vigilantes known as the Furies.

Paige accidentally summons a medieval knight, who believes Paige is the love of his life. However, the knight is under the influence of an evil sorceress.

Wanting to protect a ten-year-old orphaned boy who is a firestarter, Piper and Leo try to hunt down the demon before he can deliver the kid to The Source. Meanwhile, Phoebe is given Gram's wedding ring by Cole, but to their surprise the ring has been cursed to transform the wearer into a traditional 1950's suburban housewife.

Phoebe develops a case of cold feet over marrying Cole and casts a spell which brings a young innocent Phoebe and an old cynical Phoebe to the Manor to help her make a decision. Meanwhile, while Cole is horrified to find that The Source is now taking over his entire being, The Source's chief rival, who believes The Source is dead, is convinced by The Seer to kill The Charmed Ones in order to earn the allegiance of the entire underworld.

Piper, Phoebe and Paige come to the aid of Mylie, a Mermaid-turned-human, when she shows up looking for their help after a pact she made with an evil Sea Hag begins to go drastically wrong. Meanwhile, Phoebe attempts to divorce Cole, until he shows up claiming to want her back, Paige, attempts to juggle her new position as a full fledged Social Worker, as well as being a Charmed One, and Piper must protect her unborn baby, and face her fear of water.

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.