A string of murder-suicides have gone down in one neighborhood, and while Dyson and Hale investigate, who should they run into but Bo and Kenzi - the latter picking up some extra cash by posing as a medium who can cleanse the dark spirits from the houses. Kenzi picks up a secret passenger: A spider sneaks into her purse and wastes no time sinking its fangs into her.
Later at the pub, Hale and Kenzi watch Bo try to negotiate between Dyson and Lauren as both vie for her attention. Kenzi bows out of the party due to a splitting headache, and by the time Bo gets home, Kenzi is hearing voices and thinking everybody's out to get her. The spider that caused these symptoms has escaped the confines of the purse, and it soon finds Bo and releases the same venom into her. The two roommates are flooded with feelings of paranoia and violence, but have no idea why. They go to see Lauren for some medical relief, but the only advice she can offer is to take some aspirin and head to bed.
Dyson and Hale find unusual spider webs at all of the crime scenes and take the evidence to Lauren, though begrudgingly - her humanity and close relationship to The Ash make her suspect, in their book. She discovers that the webs belong to an Under Fae called a Djieiene, a spider whose venom produces mania and violence in its victims. Dyson realizes Bo and Kenzi have the bite, and sends Hale, with his powers of soothing and seduction, to rescue them.
But Hale isn't prepared for the fever these ladies are under. No sooner does he enter the house but Bo knocks him out cold. She can't keep her succubus powers at bay and begins sucking the life out of Hale, but the healing it produces in her snaps her awake and she stops herself before she finishes him off. Coming to her senses, she calls Dyson as she realizes the house is under quarantine, and Dyson realizes that Lauren and The Ash are responsible for this possibly deadly decision. Dyson tells her to break the thrall that Kenzi - and now Hale - are under by killing the spider, but the creature proves indestructible. Turns out the wee beast implants its heart in a much larger host - in this case, an Under Fae collector aligned with the Dark Fae - and only destroying the heart can end the spell.
Lauren wastes no time finding the collector and the implanted heart. In an impromptu and half-magical surgery, she removes the spider's hear from the man's innards and destroys it just in time to save Bo from being murdered by Kenzi. The quarantine is called off and Bo is wildly grateful for Lauren's efforts, making Dyson crazy with jealousy and distrust.