No Mercy

No Mercy is the first campaign in Left 4 Dead, spanning five chapters. The first level picks up right where the game's intro cinematic leaves off; the Survivors are told by a passing helicopter to get to Mercy Hospital to be evacuated. On their trek to the hospital they must endure Infected-filled apartment buildings, city streets, subway tunnels, and sewers, passing through Mercy Hospital until they finally reach the roof of the hospital itself to be rescued by a helicopter. No Mercy is considered the most played campaign in the entire game, both campaign mode and versus. No Mercy had some glitches that are now patched.

Below is a complete list of video walkthroughs of the 5 levels:

The Sewer Part 2
 * 1) The Apartments
 * 2) The Subway
 * 3) The Sewer Part 1
 * 1) The Hospital
 * 2) Rooftop Finale

Please know that the gameplay videos are on each campaign's info page not on each map page respectively.

Official description
The No Mercy campaign finds you on the rooftop of an obliterated apartment complex, deep in the heart of the once-thriving metropolis of Fairfield—now completely overrun with hordes of Infected. A voice from a bullhorn calls out from a rescue helicopter: "To anyone who can hear this, proceed to Mercy Hospital for evacuation". Even if it means shooting your way through narrow alleys, open sewers and seething waves of Infected to get there.

Writings on the wall
The Apartments level contains graffiti throughout the map. Much of it is located in the safe room across the street, but some is found on the level start area. "Hope" is spray painted in large letters to the left of the doorway, with "NO" written above it to change it to say "No Hope", and "Morbid" can be seen on the right wall, also in graffiti-style lettering.

In the safe room, "None of you will survive" and "Entire city is Infected, if you can read this, get out", among other things written by other survivors. In the finale, there is a written conversation on the wall talking about the duration of the change to Infected after being bitten from 5 days, 3 days, 1 day, 2 1/2 hours, then finally, to 5 minutes.

Demo
The Left 4 Dead demo consisted of The Apartments in its entirety, as well as most of The Subway. The rest of the Subway map could be seen in the demo if players went into NOCLIP mode.

Behind the Scenes
No Mercy looked significantly different in early versions; it was much more dark, but where there was light, it would be quite bright. In addition, the skyscrapers and other buildings in the background would be a part of the skybox texture, which makes them look completely flat; in the final version, the buildings dotting the landscape are actual brushes and models. The sky itself was a dark blue, compared to the blue/green mix in the final version. Buildings in the campaign would have most of their lights still on, which was changed to very few of them in the final game.

Trivia

 * There is currently a mistake on the loading screen/"movie poster" for the No Mercy Campaign; look at Zoey holding the gun, and notice her hands/fingers. Zoey seems to have three hands holding the gun on the poster. The three-handed phenomenon is caused when looking at different hand positions for the poster. The layers were just added over each other without taking one position out.
 * Another mistake includes the fact that when you're outside of Mercy Hospital, it looks complete. However, on the fourth stage, the construction looks too obvious to forget.
 * Another mistake includes the fact that when you're outside of Mercy Hospital, the (+) Mercy sign is visible. However, on the fifth stage, when you jump off the building on any side, the (+) Mercy sign is not visible.
 * One of the writings on the wall states that the government will bomb the city. This may be a possible reference to Resident Evil 3: Nemesis, in which the government bombs Raccoon City in an attempt to stop the infection.
 * In the safe room at the beginning of The Hospital, there is writing on the wall about how many zombies each survivor (that came before you and your group) killed. One of these numbers is 53,595. This is the exact number of zombie kills the player needs to get the Zombie Genocidest achievement. This is a reference to Dead Rising, in which the player must kill the entire population of the town, which comes to 53,594. Left 4 Dead one-upped Dead Rising. In the same manner, the game Prototype one-upped Left 4 Dead, in which the player must kill 53,596 enemies to get an achievement, it is possible that Left 4 Dead 2 will request you kill 53,597 Infected for an achievement.
 * You can sometimes hear the pilot of the helicopter talking to what seems to be other Survivors as he can sometimes be heard saying that he can't pick up any more passengers but is willing to drop off some supplies. The conversation can be heard in the pilot's sound files.
 * In the campaign poster, Francis can be seen wielding what appears to be a scoped sub-machine gun. This is not usable in-game.
 * This was the first campaign ever conceived.