Paige and Richard have just spent the night together and are walking into the kitchen talking of breakfast when they trip over Piper. Piper is trying to fix the garbage disposal, and Richard, in an attempt to dispel the awkwardness, fixes it using magic. Piper seems stressed as she is busy fixing all the broken things in the house. Piper seems a little concerned that Richard has started using magic again. Phoebe then walks into the kitchen dressed up and asks to talk to Piper alone. She expresses concern over Richard using magic, as last time he did he lost control, and is worried about how little Paige knows about Richard. However, Piper warns her not to meddle.
Piper is trying to unclog a sink, when a woman's face appears above the water, asking for help. The sisters orb to a pond and a woman emerges, bearing a large sword. Several demons appear, and one throws a knife, hitting the woman in the back. The sisters fight off the demons and find the sword embedded in a large stone. Paige believes the sword is Excalibur, from the Legend of King Arthur but Piper is skeptical.
Phoebe checks the Book of Shadows and can find nothing on the sword, but finds a page on Executioner Demons, demons which can be hired as assassins, and the page on the Dark Knight. Phoebe continues to be suspicious and awkward around Richard, refusing to discuss the demon with him around. Phoebe goes for her Skype date with Jason and Paige goes to her new temporary job, leaving Piper to deal with the problem. Paige tries to pull the sword from the stone, but can't.
The Dark Knight threatens a mercenary demon, who tells him that the Charmed Ones attacked. He wants "a seat at the table" as his price for hiring more assassins to take out the Charmed Ones. A large round table with a pentagram stands nearby. It seems the Dark Knight's plan is to unite all evil under the corrupted power of the sword.
Phoebe is swamped in work and unable to keep track of her column and her appointments. Paige's new temporary job is as Phoebe's assistant. Phoebe finds it weird telling Paige what to do.
Back at the manor, Piper is trying to fix the washing machine and clean around the house. A dwarf comes in and Piper realises that a line of magical beings has walked into the manor and is trying, one by one, to pull the sword from the stone. They jeer at each other's attempts. Piper tries to run them off, and calls for Chris, but Leo responds instead.
At the office, Paige is using her looks and flirting with the guys in the office to get them to do her work. Paige begins giving Phoebe advice on her column, talking about one particular question which is extremely similar to Paige and Richard. Phoebe tells her she is worried that they're moving too fast, however Paige gets upset that they are moving too fast. Just then, Leo orbs in to bring Phoebe and Paige back to the manor.
Finally, Piper gets angry and grabs the sword -- and it comes out. The magical creatures gasp and bow. A man calling himself Mordaunt appears in a vortex and tells Piper she is now the master of Excalibur.
Mordaunt tells the sister what he knows of the Dark Knight and suggests a vanquishing potion. He offers to teach Piper the use of Excalibur. Piper is annoyed, saying she does not have any time and doesn't want this power. Paige is excited, however, urging Piper to take up the power which has been lost for centuries. Mordaunt conjures up a potion recipe to kill the Dark Knight, saying that he is extremely powerful. Leo orbs Wyatt "Up There" to check with the other Elders while Mordaunt produces a sword and begins sparring with Piper.
In the demon's lair they discuss their action. An executioner demon brings in a sack which is revealed to contain a dwarf. They question him about the sword and what he knows. The dwarf informs them that Piper has the sword now and is being trained to use it. The demon demands to know who is training her. The dwarf motions he doesn't know. However, he then punches the executioner demon in his testicles, who lets go of him to grab his balls, the dwarf then escapes zipping out.
Phoebe, Paige and Richard make the vanquishing potion at Richard's place. Phoebe and Paige argue over Richard. Phoebe is worried that Paige is going into the relationship without seeing Richard and his dark side. Paige will not listen.
Mordaunt and Piper continue to train, and Piper begins to take on the power of the sword. Mordaunt states that they are becoming one and soon nothing will matter to Piper except the desire to rule. The Dark Knight and three demons shimmer into the manor. Mordaunt seems to expect them. Piper fights the Dark Knight while Mordaunt takes on the three underlings.
At Richard's place, Phoebe and Paige are continuing with the potion. Phoebe is about to add black poppy when Richard yells "NO!" and, through telekinesis, pulls Phoebe away. Phoebe has been mistrustful of Richard, but he then tosses the black poppy in, causing an explosion. Richard says, "Don't ever mix wraith essence and black poppy". He, Phoebe and Paige realize that Mordaunt wants them dead.
More demons attack, but Piper and Mordaunt kill the last of them. Paige and Phoebe orb in and try to warn Piper, however Piper is "one with the sword" and has allowed the sword's power to take over. Paige tries to orb Excalibur, however the sword deflects her orb and Piper and Mordaunt fade away -- to the Round Table with the pentagram. Mordaunt promises Piper great power, and kisses her. He reaches for Excalibur but it evades his grasp. Piper and Mordaunt have gathered several demons to the round table. They offer a loyalty oath and the demons all explode, their powers transferred to Mordaunt. Mordaunt claims Excalibur from Piper. He tells Piper that Wyatt was the true owner of Excalibur, and impales Piper with Excalibur.
As Leo heals Piper, Mordaunt shimmers into the manor and tries to stab Wyatt with Excalibur -- but it's only a teddy bear. Paige orbs back in with Wyatt, and Leo with Piper and Phoebe. Wyatt orbs Excalibur to himself and he hurls it back into Mordaunt, vanquishing him.
Piper puts Excalibur back into the stone and Paige orbs it to the attic until Wyatt is old enough to use it. Leo also fixes all the things wrong in the house for Piper, leaving her more time to spend with Wyatt.