The story kicks off when the guys visit the comic book store. While there, Stuart Bloom, the store owner, uses his artistic talent to sketch Penny and charm her into a date. Leonard is devastated and jealous, leading to two parallel storylines:
"The Big Bang Theory" The Hofstadter Isotope (TV Episode 2009) - IMDb
Despite being a "nerd," Stuart successfully woos Penny with his artistic sensitivity. Their date goes so well that Penny invites him in for "coffee" (which Penny clearly intends to lead further), but they are interrupted by Sheldon, who engages Stuart in a heated, hours-long debate about who should replace Batman if Bruce Wayne died.