It's like The Justice Society and the Justice League teaming up. Valve has just announced plans for The Passing, the first downloadable mini-expansion for Left 4 Dead 2 as the new survivors of the zombie apocalypse meet up with the four characters in the first Left 4 Dead game.

Set after the events of the Left 4 Dead 2 campaign Dead Center, the new campaign storyline is set in Georgia and will not only have the two sets of teams meeting each other, but will also include new arenas for Survival, Scavenge and Versus Modes. There will also be a new (and so far unnamed) co-op challenge mode. The Passing will also add a new melee weapon, a new firearm, and a new "uncommon common" Infected. The Passing is due for release sometime this spring. There's no word yet if The Passing will be a free download for the PC version.