The casts of both Left 4 Dead and Left 4 Dead 2 are taking their zombie-killing talents to a new, non-Valve game. A free update released today for the PC version of Zombie Army Trilogy adds all eight survivors from Valve’s co-op shooters as playable characters.

This doubles the existing character count in ZAT, bringing the total up to 16. In addition to those already in the game, you can now choose to play as L4D’s Francis, Bill, Zoey, and Louis, as well as L4D2’s Coach, Rochelle, Ellis, and Nick. ZAT supports co-op for up to four players, allowing you to mix existing characters with Valve’s additions.

Narratively, this of course doesn’t make sense, not that fighting zombies in the wake of World War II (the premise of ZAT) ever really did. But to fill in the hole of how these characters are getting together, British comic 2000 AD is releasing a short digital comic to explain everything. You can grab it for free here.

This is far from the first time Valve’s characters have appeared in other games; for example, Team Fortress 2 characters have popped up in Surgeon Simulator and, more recently, I Am Bread.

ZAT is comprised of a new campaign and remastered versions of the first two Nazi Zombie Army games. Both of those were released as DLC for Sniper Elite V2, another game from ZAT developer Rebellion. Trilogy debuted back in March 2015 for PC, Xbox One, and PS4, though today’s update is PC-only.

To celebrate the update, ZAT is 66 percent off on Steam, bringing its price down to $15.29. You can also pick up a four-pack for $45.89.