Harry is in his apartment performing yoga when Murphy comes in. She tells him that the police received reports of an explosion of green fire in a nearby park and asks Harry what he's involved in. Harry assures his friend that he had nothing to do with it but agrees to check it out with her so she can pay him as a consultant. As they talk, Murphy comments on his poor clothing and Harry points out that she always wears the same necklace. The lieutenant hesitantly explains that it was a gift from her 9-year-old daughter, Anna.
As Harry goes to get changed, Morgan slams in through the front door, his sword drawn, and tells Harry to activate his magical defenses. Murphy immediately draws her gun and Harry tries to calm them both done. Three Wardens bring in Ancient Mai, who is badly wounded. Bob hears the commotion, steps through the wall, and quickly ducks back before Murphy can see him. As they look at the windows, a wall of darkness covers the outside of the building, sealing them in and cutting the power.
One of the Wardens, Ambers, powers the lights, while the wounded Mai mutters to Harry, ordering him not to tell Murphy anything. She passes out and Harry turns to Morgan for answers. Before the Warden can respond, another of the Wardens, Soto, looks out the open door and sees a ghostly figure floating in the dark. He calls the name "Lydia" and walks to the doorway, and his body is torn apart in a burst of energy.
As Morgan secures the door, Murphy demands to know what's coming on. Harry tells her that the darkness is a force of death from the Other Side. Morgan confirms that Lydia was Soto's sister, a Warden who died in the line of duty. Murphy insists that she can't deal with the magic, and Morgan tells Harry that they need to secure Mai. Harry directs them to his upstairs bedroom and then demonstrates magic to Murphy, activating the normally invisible protective sigils inside of the shop.
When Morgan comes back, he refuses to discuss the situation with Harry because Murphy is there. He explains that they only came to the shop because it was close, and asks Harry if he's going to help. Harry reluctantly agrees and they go to check out the back. Morgan talks about wiping Murphy's memories once the crisis is over, and Harry slams him into a pillar and insists that they won't do anything to his friend. Shocked at his own anger, Harry manages to control himself and suggests that they try to cut through the darkness barrier. Morgan suggests that they create a coherent burst of light and Harry agrees. They go to the back room and Harry casts a spell on a crystal. Morgan pensively admits that he trained Soto and blames himself for the young man's death. They then combine the magical forces of their staff and sword, focusing them through the crystal. However, they fail to cut through the darkness and the backlash sends them both flying.
As Murphy runs back to investigate the noise, Bob emerges from the wall, startling the lieutenant. The spirit draws Harry aside and warns him that whoever cast the barrier spell did it from inside the shop. Harry already knows, having confirmed that the sigils were active earlier. Bob confirms that the barrier is everywhere, even beneath the floor, and there's no way out. Stumped for the moment, Harry asks Bob to lie low and watch everyone to figure out who the traitor might be.
As Bob slips away, the darkness seeps in through the back door, covering the room. Morgan and Harry retreat to the front, and Amber insists that they tell Harry the truth. Murphy agrees, pointing out that they're all in danger, and Morgan reluctantly explains that Mai was meeting someone at nearby Ecker Park. Someone from above launched an attack of green fire, but before he can explain further, Mai staggers out and tells him to stay silent. Harry objects but Mai says that he's just like his mother, always trying to change things that shouldn't be changed. The remaining Warden, Wallace, takes her back to Harry's bed.
As Morgan and Amber try to work out a spell to penetrate the barrier, Harry asks Morgan to continue what he was saying earlier. The Warden refuses and Harry notices that Murphy is opening the front door. He quickly pulls her back and she says that she saw her father in the darkness. Harry reminds her that her father is still alive and living in Florida, and Murphy admits that she hasn't talked to him in over a year and wouldn't know for sure if he was dead. When Murphy complains that she feels useless, Harry points out that she was the one to convince Morgan to talk as much as he did. He suggests that she use her detective training and figures out who among them is the traitor.
As they talk, Harry notices that the darkness isn't seeping through the front door like it did in the back. He talks to Morgan privately and suggests that the Warden teleport out. Morgan demonstrates that he can prevent people from perceiving him but he can't actually teleport. Seizing on the idea of invisibility, Harry creates seven shielding bracelets and suggests that they just walked out, cloaked from the darkness. However, the bracelets will only last a few seconds and they have no idea how thick the barrier is. Morgan suggests that they send Bob out to investigate since he's already dead. Bob isn't happy with the idea but agrees to help. As they go to the door, Bob tells Harry that he hasn't found out anything by watching the others. Harry promises to summon him back and Murphy thanks Bob for being so brave. The spirit steps out into the darkness but he doesn't return, and Harry is unable to summon him back.
Harry goes to the back door and tries to see out into the darkness, looking for his friend. Murphy approaches him just as Harry's father Colm appears out of the darkness. He tries to shout a warning, but Murphy warns Harry that it's some kind of trick. The spirit disappears and Amber calls them to the front to where Bob has come back in. Bob tells them that the beings lurking in the darkness are far more powerful than him and that he was lucky to escape them and return. When Morgan asks him how thick the barrier is, Bob tells him that there is no barrier. Whoever is responsible shifted the entire building to the Other Side, effectively trapping them in Hell.
As they try to come up with another approach, Morgan puts on one of Harry's jazz records. He talks with Murphy, who wonders how long he's known Harry. Morgan explains that he first met Harry just after Colm died, and that he's always been trouble because of the Morningway blood. Realizing that he's said too much, Morgan tells Murphy to be quiet so he can think of a way out.
Amber is looking in the refrigerator for food when Harry approaches him. She tells him that she's not the traitor even though she and Wallace are the only people that Harry doesn't know. When Harry assures her that he's considering everyone, Amber figures that he's the traitor, pointing out his reputation for using black magic to kill his enemies.
Harry goes to the workshop to figure out a spell without success and finally starts smashing things in frustration. As he and Bob works, the darkness starts seeping in and Colm steps out of it. While the ghost looks around in apparent surprise, Bob tells Harry to stay away from it. Colm agrees with Bob, and Harry asks Bob to give them a minute alone. Once they're alone, Colm says that "she" sent him there to warn that there is a traitor among them. Harry tells his father that he already knows, and Colm tells him that the only way to be safe is to kill everyone in the building, guilty and innocent alike. Before he can explain further, something pulls him back into the shadows.
When Harry goes back to the main room, he hears Amber yelling at Wallace. Wallace throws Amber off the second floor loft and then jumps down, stunning Murphy with a magical blast. Morgan tells him to back off but Wallace draws his sais and says that he has to kill everyone to make sure he gets the guilty party. He attacks Morgan, who defends himself, while Amber tells Harry to check on Mai. When Harry goes upstairs, Mai is barely conscious and warns him that someone is keeping her from magically healing... and that someone has to be close. She glares at Harry and tells him to work harder to protect her. As Mai passes out, a shot rings out. Harry looks down to discover that Murphy has shot and killed Wallace. Harry goes downstairs and Murphy, shocked wonders why she shot the Warden. He assures her that Wallace was trying to kill them all and she did what she had to.
As Morgan brings Mai downstairs with the others, the darkness sweeps across the upper loft. They try to work out what's going on and Harry realizes that Mai is the one holding it at bay. Every time she grows weaker, the darkness grows closer. Harry figures that Mai is the one who shifted the building, retreating so that she could heal, and her powers have been holding the darkness at bay. Morgan points out that she isn't healing and Harry figures that whatever is with them is blocking Mai's abilities.
Harry realizes that there's only one person among them that he invited in and could be the traitor... and turns to Murphy. She smiles smugly and tells Harry that it took him long enough, and then breathes a burst of green fire at him. Harry blocks it just in time and Morgan creates a barrier wall across the room, sealing their attacker out. The intruder tells Harry that Morgan knows what it is and that Harry has chosen the wrong side. It explains that Mai only wants power and wouldn't help Harry if the opportunity arose. Morgan denies it but the creature offers Harry his life in return for Mai's corpse.
Morgan finally admits that they're facing a dragon, which can shapeshift and lives immortal. The drake drops its illusory disguise and tries to smash through the shield, and Harry tells Morgan to contract his shield wall to just protect them. He then goes to Mai and uses magic to make her sicker. The darkness sweeps forward, consuming the drake, and Harry, Amber, and Morgan concentrate all of their magic into healing Mai. Once she recovers, she heals herself and returns the building back to its correct location.
As the surviving Wardens prepare to leave, Amber apologize to Harry for misjudging him. Mai casually starts to walk out and Harry asks if that's all there are given that two people died. Mai explains that she knew that the drake had slipped inside and was willing to kill all of them, including herself, to eliminate it. Harry points out that Mai is also an immortal shapeshifter, but avoids asking about herself. She congratulates him on having some common sense and leaves.
Murphy arrives just as Morgan and the others are leaving, and Morgan greets her. Once they're gone, Murphy barges in and tells Harry that there was an explosion of green flame. She asks if he had anything to do with it and Harry says that he doesn't. When Murphy wonders why Morgan and the others were there, Harry says that they're better off knowing. She suggests that they get some Chinese food for lunch and Harry hastily offers Greek instead.