Harry is taking his test for a PI license so he can continue working as a consultant. However, he's distracted by the female teacher's assistant, Liz Fontaine. The male teacher, detective Jim Brennan, tells them that anyone who can answer his next question will pass regardless of their score. Brennan asks what is the sacred rule of the private investigator. When no one answers, Brennan calls on Harry, who manages to guess that the right answer is "Keep your mouth shut." Brennan tells them that after 40 years, it's the most important rule he's learned. As Harry takes a break with the others, Liz approaches him and asks for his tuition. She warns him that he'll have to leave the class if he doesn't pay up. Harry explains that he can't make money unless he gets the permit, but Liz is unimpressed.
Later, Harry goes to see Murphy at the station to ask her for a job. She warns him that she can't give him a case until he gets a permit, and refuses to give him a loan. Murphy finally agrees to give him $250 and let him make up the rest. Harry goes to see Brennan and determine if he'll take that much. When he enters Brennan's office, he finds the PI dead at his desk, an open bottle of whiskey in front of him. Harry touches the body and a bolt of lightning passes through him. As he screams in pain, Liz comes in with coffee, sees her boss dead, and draws her gun on Harry.
The police are called in and secure the crime scene. Murphy warns Liz that they can't arrest Harry for murder and the assistant storms off. She then tells Harry that it looks like Brennan died of a stroke and Liz confirmed that her teacher had heart problems. When Harry wonders what Liz's story is, Murphy explains that she hired Brennan to track down her son five years ago, and she has a criminal record for embezzlement and fraud. Harry avoids telling Murphy that Liz pulled a gun on him, violating her parole, and Murphy asks if he can decipher the code that Brennan used on his most recent case. Liz didn't know anything about the case and claims she's never seen the code. Harry agrees to do it for free
Later, Harry has Bob look at the cipher. The spirit admits that it's a private code and Harry offers him the piece of chalk that Brennan used in class as the basis for a translation spell. As Harry prepares the spell, Bob wonders why he's getting involved, pointing out that they have no client and no pay. Harry figures that Brennan deserves it for being a good detective but Bob points out that he finds Liz attractive. Once the spell is ready, Harry drinks it and then breathes fire onto the words. They translate into a note saying that Brennan was running surveillance on a R. Jones and the client's suspicions were unfounded. They write down everything after it fades away.
Harry goes to the address and discovers that Ronald Jones is the director of human resources for a meat packing company. As he arrives, he sees Liz outside the office building and she demands to know why he's there. Harry explains that he doesn't think Brennan died of natural causes. Liz says that she know he didn't because Brennan quit drinking 13 years ago and he kept the whiskey bottle around as a reminder. She figures that Brennan left the open bottle as a signal to her that something was wrong. Liz tells Harry to stay away but he refuses, saying that he liked the guy and took the murder personally.
Liz agrees to let Harry tag along and they go to see Jones. He claims that he doesn't know Brennan and Liz tries to question him. Harry keeps interrupting her and asks if the attractive woman in a photo with him is his wife. He hesitates and then says that it's his wife, Felicity, and Harry says they're done. However, Jones finally admits that he hired Brennan to follow Felicity. They assure him they can be discrete and Jones explains that his wife was spending a lot of time out. He thought she was having an affair and hired Brennan, and Brennan assured him that Felicity wasn't having an affair and what he found out saved their marriage. Liz wants to know where she was going, but Harry quickly gets them out.
Outside, Liz plans to follow Felicity because she figures a HR director couldn't attract a beautiful woman like her. She suspects that Felicity or her boyfriend killed Brennan, but Harry warns her that she's making a lot of unfounded assumptions. Liz gets into Harry's car, starts driving, and says that since he doesn't have any ideas, they'll go with her theory until he comes up with something better.
The two detectives follow Felicity as she goes shopping and buys a piece of lingerie. Harry is sure that she's happy with Jones but Liz doesn't believe it. As they look at photos of the couple's expensive home, they see Felicity go into a building that isn't a store. As Harry crosses the street, he gets a sudden flash of energy and realizes that the building is built on a cluster of ley lines. Meanwhile, Liz confirms that the building contains the Overland Fertility Clinic. She figures Jones wants it confidential so no one learns that he's infertile. Harry says that it's a dead end and starts to leave, but Liz figures that he's going to go in anyway. She points out that he'll need to be with a woman to make it look convincing and Harry reluctantly takes Liz in. As they go in, Liz gives Harry Brennan's camera.
Harry and Liz meet with Dr. Overland, who asks them personal questions about their sexual intercourse. While Harry claims they have a lot of sex, Liz blames the infertility on him. Overland asks Harry to provide them with a sperm sample and Harry goes to a private room. As he leaves Overland's office, he senses another ley line leading to an alternative therapy room. He quickly enters the room and finds a room filled with candles, magic symbols, waxwork figures, and a bed at the center. Harry puts some samples of leaves from a bowl into the specimen jar and uses Brennan's camera to take some photos. As he does so, he notices a metal plate with mystic runes on it, suspended from the ceiling above the bed.
A nurse, Anya Petofsky, brings Felicity in and warns her that it make take repeated treatments. Harry steps out and claims that he's lost, showing her the specimen jar. He asks her what goes on there but Anya simply says "treatment" and gives him directions to the private room. Harry catches up to Overland as he escorts Liz out of his office, and Harry says that he wasn't feeling motivated. He quickly leaves with Liz and outside, she tells him that Overland was overly grabby. Harry gives her the camera and shows her the tea leaves. Liz figures that it's a dead end but Harry isn't convinced. They split up to check out the tea and develop the photos, and Liz agrees to meet Harry later at his apartment.
Back at his apartment, Harry has Bob analyze the tea. The spirit explains that the leaves diminish inhibition and memory while heightening spiritual energies. As Harry describes the plate, Liz arrives with the photos. However, she's done some checking on Harry and points out that he bills himself as a wizard. Liz wonders what that means but Harry would prefer not to discuss it. She points out that Brennan had a lot of other pictures on the camera that he took when he hid it in the camera. They show Anya performing a ritual with Felicity, and Liz figures that Harry believes Overland and Anya had something to do with Brennan's death.
As they talk, someone throws a bundle of dynamite through the window and drives off. Harry uses his hockey stick to contain the blast and then collapses from the strain. As Liz runs to him, Bob comes out and Harry does the introductions. He then tells Liz that the case is about much more than an affair and tells her to drop the case. As they talk, Bob sees the photo Harry took of the plate. Meanwhile, Liz promises to find the killer of the one man who treated her like a human being. Bob asks to talk to Harry privately and tells her that the symbol is a doorway for incubae. Incubae are sexual tormenters who prey on women, driving them mad with lust and desire. Bob points out that a fertility clinic would be the perfect place for such a demon to find desperate women. Meanwhile, Liz confirms that Overland had his medical license revoked in California, and Anya has been at the clinic two years. Before that, the clinic was barely getting by and now they're making a lot of money.
Harry and Liz break into the fertility clinic and go into the therapy room. Overland is on the couch, dead, and Harry confirms that the same energy that killed Brennan killed Overland. Harry notices a needle mark on Overland's neck and an acupuncture needle lying nearby on the floor. He remembers Anya using the needles earlier and explains to Liz that she overloads her victims' nervous systems. Liz is eager to find Anya but Harry points out that they should call the cops just like Brennan would have done. When he points out that she has to be careful, Liz wonders what Murphy told him.
Murphy comes in with a squad to secure the clinic. She tells Harry that Brennan had a needle mark in the neck and didn't die of a stroke. However, Murphy is less than happy that Liz is apparently one step ahead of her. Harry explains that Overland died the same way and that Anya is involved. Liz persists in taunting Murphy, who threatens to throw her in jail and tells Harry to get the woman out of there.
Back at the apartment, Bob is going over the photos and sees a faint psychic smudge on one of them. He touches it and a wormlike creature briefly possesses his spirit body.
Harry and Liz come back and Harry warns her that she can't talk to cops like that. Liz complains that she wants a beer, even though she's on the wagon, and Harry gets her a glass of water. They try to work out why Anya killed Overland and who threw the bomb, and wonders if they need to warn Jones. As Harry gets the water, Liz looks at the photos of the Jones home and gets an idea.
In the kitchen, Bob warns Harry that Felicity is being used as an incubator for an incubus child. He shows Harry the photo from earlier and how the smudge on it moves. Bob explains that it's a trapped spirit captured on film, ensnared when it traveled from one world to the other. He figures that the incubus father seduces Felicity and then implants a piece of his soul with Anya's help. The tea treatments unlock Felicity's spiritual energies and let the incubus child feed on her until it can live on its own. Brennan's camera captured it as it was moving to the Other Side where it would have gained corporeal form. That leaves the question of who is the father. Harry realizes that Overland hired Brennan rather than Jones, to investigate his own clinic when he realized something was wrong.
When Harry goes out to talk to Liz, he discovers that she's left. He notices the house photos and realizes where she's going.
At the Jones house, Anya is preparing to give the sleeping Felicity another "treatment." She inserts the needle into Felicity's abdomen but then hears someone moving outside the bedroom. A minute later, Liz comes in and sees Felicity on the bed. As she tries to wake the woman up, Anya tries to stab her from behind. Liz spots her and they struggle, and Liz head butts Anya unconscious. She prepares to shoot Anya, figuring that she killed Brennan and threw the bomb. However, Jones comes up behind Liz and entrances her with his sexual energies. He tells Liz to confess to the killings so that Anya can fulfill his dream, but first he wants her to kill Harry. Enraptured, Liz immediately agrees to do whatever Jones wants.
Harry enters the house and finds Liz with Jones. She aimed her gun at Harry, who tries to explain to her what's going on. He reminds her that Jones killed Brennan but she ignores him and prepares to fire. Harry offers Jones a deal: Felicity and Liz's release in return for the photo with the captured spirit. If Jones doesn't take the deal, Harry will burn the photo. Jones tells Liz to fire and Harry deflects the bullets, and then sets the photo on fire. He throws it on the phone and Jones leaps for it, and Harry hits him with a magic blast. The incubus' physical form is disrupted and he drifts through the portal to the other side.
Liz recovers but doesn't remember much, but is happy that she solved the case. When she wonders what they'll tell the police, Harry suggests that they tell Murphy that Anya and Jones were running a fertility scheme out of the clinic. Everything else will be the truth, other than glossing over how Jones "left town." Felicity wakes up and wonders where her clothes are.
Sometime later, Liz is packing up Brennan's office when Harry comes in. He suggests that she pick up where Brennan left off, but Liz points out that she's a convicted felon and couldn't get the permits. Harry offers to put her in touch with some people and says that she should take the chance for happiness. Liz agrees and Harry asks her to steer some business his way. As Harry leaves, he magically changes the name on the door from "Jim Brennan" to "Liz Fontaine."