Episode 71 of "Angel"
s04e05 Supersymmetry

After Fred's physics article is published... she's invited to speak at a symposium where a portal opens, bringing her face to face with a demon and a dark secret about her old professor. Meanwhile Angel visits a comic book store to try and solve the mystery and find out why someone would want Fred out of the way. While doing so, he discovers that he is a very popular character in online chatrooms.