The Young Hollywood crew heads to San Diego and the Mecca of geekdom, Comic-Con, where they run into "Shannara Chronicles" star and MTV Fandom Awards host Austin Butler, hang out at Nerd HQ with Zachary Levi, chat with stars of "The Vampire Diaries" on the press line, and team up with the cast of Nickelodeon's "All That" to take the ultimate "Double Dare" challenge.