Master, if you need me I'll be in the west library. Weeping. 
Superman/Batman #51 is an issue of the series Superman/Batman (Volume 1) with a cover date of October, 2008. It was published on August 20, 2008.
Synopsis for "Lil' Leaguers (Part I of II)"
Air Force One is in trouble once again. This time however, disaster is averted not by Superman, but by Lil' Superman, a pint-sized version of the Man of Steel. A similar phenomena takes place in Gotham City when a miniature version of Batman takes down two thugs. Before long, the real Superman and Batman encounter their smaller doppelgangers and try to understand exactly who they are and why they are here on Earth. Within minutes, the mini-heroes are greeted by their colleagues, the Lil' Leaguers. Determined to keep this bizarre situation contained, Batman recommends that everyone should relocate to the Batcave so they can get this mess sorted out. As the Lil' Leaguers run amok through the Batcave, Superman and Batman discover the true architect of their arrival - Mister Mxyzptlk. Mxyzptlk felt that Superman and Batman have been going through too many dark times, and needed a little light-hearted fun to brighten their spirits. He neglects to tell them however, that miniature versions of their greatest adversaries have arrived on Earth as well.
Appearing in "Lil' Leaguers (Part I of II)"
Featured Characters:
Supporting Characters:
Antagonists:
- Floronic Man
- Mister Mxyzptlk
- Lil' Bizarro
- Lil' Brainiac
- Lil' Catwoman
- Lil' Clayface
- Lil' Cyborg Superman
- Lil' Doomsday
- Lil' Hush
- Lil' Joe Chill (Flashback only)
- Lil' Joker
- Lil' Killer Croc
- Lil' Lex Luthor
- Lil' Mister Freeze
- Lil' Poison Ivy
- Lil' Two-Face
Other Characters:
- Lil' League (First full appearance)
- Lil' Batman
- Lil' Black Canary
- Lil' Black Lightning
- Lil' Flash
- Lil' Green Lantern
- Lil' Red Arrow
- Lil' Red Tornado
- Lil' Supergirl
- Lil' Superman
- Lil' Vixen
- Lil' Wonder Woman
- Lil' Zatanna
- Lil' Kryptonians (Flashback only)
- Lil' Jor-El (Flashback only)
- Lil' Lara Lor-Van (Flashback only)
- Lil' Martha Wayne (Flashback only)
- Lil' Thomas Wayne (Flashback only)
- President of the United States
Locations:
- Gotham City
- Batcave
- Lil' Crime Alley (Flashback only)
- Wayne Enterprises
- Lil' Krypton (Flashback only)
- Metropolis
- Washington, D.C.
Items:
- Batcomputer
- Lil' Batrope
- Lil' Green Lantern Ring
- Lil' Lasso of Truth
- Lil' Lex Luthor's Warsuit
- Lil' Mister Freeze's Ice Cream Gun
- Lil' Trick Arrows
Vehicles:
- Air Force One
- Batmobile (Cameo)
- Lil' Superman's ship (Flashback only)
Notes
- This issue is reprinted in Superman/Batman: Finest Worlds, Superman/Batman Vol. 5, Superman/Batman Omnibus Vol. 2 and World's Funnest.