The episode picks up after Karen has been afflicted by a supernatural "Wreathe-Fire Bee" oddity.
Watch the episode to see Koyomi's race against time to save Karen from her supernatural fever: Nisemonogatari Karen Bee, Part 4 - Watch on Crunchyroll Crunchyroll• Jan 28, 2012 Nisemonogatari Episode 04: Karen Bee, Part 4
Koyomi Araragi returns home to find Karen suffering from an intense fever—a symptom of the bee's sting. He discovers that Tsubasa Hanekawa has been secretly cooperating with his sisters to track down the source of a curse spreading among local middle schoolers.