Throughout the course of the series, one of the running subplot lines involved Dr. Natalie Lambert's research into the vampire condition with the hope of finding a cure for it so that Nick Knight could become mortal again.
This plot line was introduced in the series premiere, "Dark Knight", when Natalie stirred up a noxious-looking bright green liquid and offered it to Nick as a substitute for the blood that he normally drank. Under some pressure, he reluctantly took a sip—and promptly spat it out. This set the pattern for a series of similar incidents over the course of other episodes of the show.
In most episodes, Natalie's attempts to treat Nick were treated as comic relief, despite their serious import to his life were she to succeed. As such, they were little more than a running gag—except in "The Fix". In this one episode, a potential cure offered by the drug Lidovuterine-B became the subject of the main plot.