Update: In an e-mail exchange with Kotaku, Valve’s Chet Faliszek gave some details on PAYDAY: No Mercy. Here they are:

“This is a 100% Overkill made mission for Payday: Heist simply set in no mercy hospital from the Left 4 Dead series. It is not telling the origins of the Infection and is not canon. There is a little cameo from one of the L4D characters (the comic should give you a hint) and a fun little easter egg with a payoff later in Left 4 Dead. Just us having some fun with a studio we are friends with and who have also made a great co-op game.”

Interesting. He also adds that the original trailer’s appearance on YouTube, then its removal 1-2 hours later, was nothing more than a bad case of “just some over-excited marketing guys.” Oh well.

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As we can see from the video, this cross-over will be a new heist level for PAYDAY, taking place in Mercy Hospital (one of the initial sites where infected individuals were admitted), as featured in L4D1’s first campaign: No Mercy. We can also see that this is all clearly taking place before the zombie outbreak (but after the Green Flu virus began to spread) and, in fact, the original video’s description (“Have you ever wondered how the Left 4 Dead series began? It started with a heist!“) implies that this hospital heist will show how the L4D outbreak really started. Which is… interesting, I guess.

In the trailer, there’s a voice line from Bain, the PAYDAY heist crew’s off-site operator, which sheds some light on the exact nature of the heist: “We’re here to draw some blood from a guy that they have in quarantine.” He then adds: “This should be quick and clean.”, before the trailer cuts to… shots of the four-man crew, engaging law enforcement in some pretty heated firefights. Ouch.

Later on, he also says: “I’m sorry guys, but I’m out. You are left for dead.” Still, what exactly led to the operation going haywire seems unknown. Interestingly, no zombies are shown in this trailer – it’s just the four robbers, engaging SWAT and FBI forces in armed combat. Maybe they’re saving the zombies for some sort of surprise ending, but I don’t know.

PAYDAY fans will also notice the new game equipment shown in this trailer: the Glock automatic pistol, and the flashlights. This is all-new stuff that has never been introduced in PAYDAY before – I wonder how this DLC release will be distributed exactly – whether it’ll be part of the upcoming paid Wolf Pack DLC for PAYDAY, or whether it’ll be a separate (perhaps free) DLC release altogether. Well, however it goes, I’ve got to say that it does look very cool. More of PAYDAY’s intense and action-packed gun battles, combined with the Left 4 Dead fiction… really, what’s not to like? I am definitely looking forward to hearing more about this thing soon.

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And in other Left 4 Dead news: Today, Valve released another one of their regular Mutation updates for Left 4 Dead 2, including a brand new in-game Mutation for Survival Mode, as well as an old insight into Valve’s decision-making philosophies for the Infected AI and dialogue systems.

Pretty cool stuff – Check it out.