Sheldon's emotional and mental superiority is called into question when the gang successfully incapacitates him with a Halloween trick. He implements numerous ploys to exact frightful vengeance upon them.
Meanwhile, in the main plotline of the episode, Leonard is hit on at the comic shop by an attractive girl, Alice, who seems to be interested in him. He seeks advice from both Penny and Sheldon, who don't prove to be particularly helpful on the matter. He finally admits to Alice that he has a girlfriend, but not until he's already made out with her a couple of times. Though he feels that he has maintained the moral high ground in the situation, Alice doesn't quite see it that way.
To maintain the moral high ground, he admits to Priya what has happened. She's not upset about the situation, because it happens to everyone ... including her. A couple of weeks earlier, it seems she had slept with an ex-boyfriend.
The episode itself is relatively weak on plot, but makes up for it with some great jokes based upon the various attempts at Halloween pranks.
This Episode's Science
Physics Equations - The episode begins with Sheldon working on a series of equations on a dry erase board. He mumbles some comments about what he's doing, starting with "Reverse the spin on the antiproton." The antiproton is the antimatter particle associated with the proton. The spin refers to the quantum spin on the particle, which is an inherent trait of the particle similar to angular momentum. It can have either a positive or negative spin, which is how he is able to "reverse" it in his equations.
Halloween Science - When Sheldon has a Halloween prank played upon him, he believes that the blood oozing from the wall is "a phenolthalein indicator exposed to a sodium carbonate solution" (explained over at About.com Chemistry in the article "How to Turn Water into Wine or Blood") and identifies a glowing skeleton as a "phosphorus skeleton." Details on these sorts of tricks can be found at "Top Halloween Chemistry Projects."
Leonard: And reverse the spin on the antiproton. And gamma becomes alpha. Multiply by a matrix of negative i comma zero ... and there we have it. Conclusive proof that I'm absolutely worthless after nine o'clock.