Mine was "The Cure for What Ails You" first ep in TUFS...Oh wow. I wasn't even looking for fanfic when I came across TUFS. I didn't even know such a thing called fanfic EXISTED and this was like 4 years after the show ended that I just happened to be going through withdrawl, so I started surfing the web and I got sucked in. And I darn near fell off my chair when I found the Archive. I thought I'd died and gone to heaven...
