Left 4 Dead 2

Plot

Like
Left 4 Dead
, the sequel involves the aftermath of an apocalyptic pandemic and it is 1 week after the first game. There has been an outbreak of a rabies-like pathogen that causes infected humans to behave like zombies. The four survivors have to fight their way through the hordes of infected, using safehouses along the way to rest and recover, in order to reach extraction points.
Left 4 Dead 2
is set in the Southern United States, starting in Savannah, Georgia and ending in New Orleans, Louisiana.
Left 4 Dead 2
introduces four new survivors, whose backstory will again be provided through dialogue. Unlike the first game where there was no significant story development,
Left 4 Dead 2
will feature a story arc.
After climbing a hotel in Savannah to achieve rescue, the survivors find themselves abandoned by helicopters, and make their way to the local mall after hearing word of a second evacuation point there ("Dead Center"). The mall proves to be overrun, and the four use a stock car to bust out of the mall and travel towards New Orleans. Finding the highway completely blocked by wrecked vehicles, the four are forced to travel through a still-operating amusement park, and start a huge fireworks-and-lights show in order to attract the attention of a helicopter pilot ("Dark Carnival"). Though rescued, they find their pilot to have been infected, and are forced to kill him, causing the helicopter to crash into a bayou ("Swamp Fever"). Finding brief shelter in a plantation mansion, they make radio contact with a boat captain named Virgil who rescues them, but informs them that he needs additional diesel fuel to make it to New Orleans. Amid a torrential downpour, the survivors go ashore, make their way to a gas station to get fuel supplies, and return and defend their position while waiting for the boat ("Hard Rain"). Virgil takes them to New Orleans, where the military appears to still be evacuating survivors, but as they make their way to the extraction point, they find that the military is actually bombing the city to cleanse the infection ("The Parish"). They are able to clear the bridge amid a mass of infected and reach safety, moments before the bridge is demolished.

Gameplay

Like its predecessor,
Left 4 Dead 2
is primarily a first-person shooter with a heavy emphasis on cooperative gameplay, with some cutscenes presented in the third-person or using pre-rendered movies. The game presents five new campaigns, each composed of five smaller maps. In the first four maps of any campaign, the survivors attempt to reach a safehouse, while the final stage requires the survivors to call for rescue and hold against a large horde of infected while they wait.
Each survivor can carry one main weapon and either one or two pistols or one of several melee weapons introduced in
Left 4 Dead 2
such as frying pans or chainsaws. Though melee weapons will cause extra damage to the infected when struck, the survivors can use any other weapon or item for weaker melee attacks and to temporarily push the infected back. They also carry a flashlight, which can be used to maneuver in dark areas but may startle the infected; a first aid kit or defibrillator kit which can be used to heal or revive other survivors respectively; pain pills or adrenaline shots that give a temporary burst of health recovery and agility; and a throwable weapon—a Molotov cocktail to set an area on fire, a pipe bomb modified with smoke alarm to attract a large number of infected before detonating, and new to
Left 4 Dead 2
, a vial containing Boomer bile, which can be thrown at infected to cause them to turn on each other. A supply of these weapons and items and additional ammunition are generally found at the start of each campaign and within safehouses, but may also be found scattered about the level.
As the survivors make their way through the campaign, they must work together to make their way to the safehouse and rescue. Players are shown the health and status of their fellow survivors, and should they not be in sight, will also be shown the silhouette of the other survivors. Taking damage from the infected will cause the survivor to lose health, and their actions will become slower as more damage is taken. If a survivor takes too much damage, they will become incapacitated, and forced to fight off the infected using pistols until rescued by another survivor or dying from further damage; the character remains dead until the next level, or, in Campaign mode, reappears in a "rescue closet" to be freed by the other survivors. Many of the special infected can quickly finish off a survivor if they are not rescued, and thus the game encourages players to stay as a group and work together to traverse the level safely. Should all the survivors be killed or incapacitated, the game is over, and players will have to restart that level. While most events the players experience are semi-randomly generated by the game's AI Director, some levels feature staged events which, when triggered by a survivor, will cause the players to be rushed by a horde of infected.
Left 4 Dead 2
retains the three game modes of the original game—Campaign, Versus, and Survival—and adds a new game mode called Scavenge. In Campaign mode up to four human players fight against the computer-controlled infected to make their way between safe houses and eventually to rescue; any survivors not controlled by a human player is controlled by the computer. In Versus mode up to four other human players take control of the various Special Infected who try to prevent the Survivors from reaching the next safe house. The two teams swap sides once per chapter and are scored based on their play through as Survivors, with the scoring based on factors such as health, distance traveled and the number of Survivors alive at the end. Survival mode is a timed challenge where the survivors are trapped in a section of the campaign maps, and try to survive as long as possible against an unending onslaught of infected. In the new four-on-four Scavenge mode, the survivor players are required to collect and use as many of the sixteen gas canisters scattered about a level to maintain fuel in power generators, extending the time in the round, while the infected players attempt to stop them, or at times, detonate the canisters themselves. New maps specific for this mode have been distributed with the game.[9]
The game also features a "realism" mode, which can be enabled at any difficulty for any of the game modes. The realism mode removes some of the video game artifacts from the gameplay: survivors will not be able to see the silhouettes of their teammates, and should they die, they can only be revived with a defibrillator kit and will no longer respawn later in the level. Damage dealt to Infected is also changed, with headshots dealing more damage to enemies, rather than limb or body shots, making gameplay even more of a challenge. The realism mode, which is designed to force players to work closely together and rely on voice communication, was created to give players a way "to be challenged as a team" without having to increase the difficulty level of the game, according to Valve's Chet Faliszek.

Survivor characters

Left 4 Dead 2
features a new cast of human survivors,

• 
  Coach
  

Coach has a big heart, a healthy appetite, and a wicked swing with a chainsaw. After a knee injury ended his career as a defensive lineman in college, Coach salvaged his Physical Education degree (barely) and landed a job teaching health at the local high school in his hometown of Savannah.
Working as a defensive coordinator for the freshmen team might not have been the best path to a pro coaching career; but it’s come in pretty handy in guiding a group of Survivors to safety. Coach has watched his beloved hometown get ravaged by infected—now it’s time to deliver his own brand of Southern hospitality.

• 
  Rochelle
  

As a low-level associate producer for a big-name news station, Rochelle's job mostly consisted of lugging cables and fetching coffee. But when the outbreak hit and staff started calling in sick, Rochelle got her break: producing a segment from Savannah about the evacuation center located there. She was still setting up the cameras when her big story became a war zone.
But that doesn't mean she's letting go of her dream job yet. Surviving a zombie apocalypse is just something else she can use her wits and drive to produce the hell out of.

• 
  Ellis
  

Ellis is a mechanic with a love of life, a firm belief in his own immortality, and the ability to treat any setback as a fun dare to impress his friends. Born and raised in Savannah, Ellis divvies up his time working at the local garage, hanging out with his buddies, and dropping by for Sunday dinners with Mom—why’d anybody want to live anywhere else?
Then the zombies had to go and spoil it. Now Ellis is looking for new things to occupy his time, and finding plenty. It turns out the zombie apocalypse is one big dare, and there’s no shortage of crazy stuff he can try to impress his new buddies.

• 
   Nick
  

A lifetime of drifting from city to city, finding back-alley card games and trying to stay out of jail has taught Nick two valuable lessons: Don’t trust anyone, and look out for number one.
He’d come down to Savannah looking for some gullible fish on the riverboat gambling cruise circuit. Instead he found a city about to be engulfed by infection. Now he finds himself forced together with three complete strangers in a fragile alliance that goes against every instinct he lives by. But he’s going to have to learn to trust them if he wants to survive.
While the game is intended as a continuation of the original, occurring a week after the first game begins, Valve decided to create a new group of survivors because of the change in location. In addition to the four playable characters,
Left 4 Dead 2
also features a support character in the form of Virgil (voiced by
Randall Newsome
), a Cajun boat captain, who appears in the game's later three chapters. This differs from the original game, where NPCs made little more than a single appearance.

Infected characters

The infected in
Left 4 Dead 2
are largely unchanged from
Left 4 Dead
. While referred to as zombies, the infected are humans that have been infected with a mutated strain of rabies. The most numerous infected encountered by the survivors are the "common infected", individually weak, but can swarm and overwhelm the survivors with large numbers. In
Left 4 Dead 2
, damage dealt to the infected will be portrayed more realistically, with bullets tearing off bits of flesh and in some cases, limbs. A new addition to
Left 4 Dead 2
are the "uncommon infected" unique to each campaign. By virtue of equipment worn before infection or mutation, they possess an ability that separates them from the common infected. For example, in the Parish campaign the player will encounter infected wearing Hazmat suits and riot gear, making them almost impervious to fire and gunfire from the front, respectively. Also, in the campaign "Dark Carnival", there is a clown uncommon infected that summons a horde of zombies, similar to the boomer bile. However these hordes appear to be greatly reduced in size compared to the boomer bile hordes.
As in the first game, there are "special" or "boss" infected in addition to the common infected whose mutations grant them special attacks that make them much more dangerous. The presence of such infected nearby is hinted at by sound effects unique to each type, or with certain musical cues. The five special infected from the first game return in
Left 4 Dead 2
, some with modified behavior. They are: the Boomer, a bloated infected whose vomit and bile (which may be released at will and upon death) blinds the player briefly and attracts a horde of common infected; the Hunter, an agile infected that can pounce on survivors from great distances; the Smoker, an infected that can ensnare survivors with its long tongue from a distance and, upon death, releases a cloud of smoke that obscures vision; the Tank, a gigantic, muscular infected that is powerful and difficult to kill with the abilities to knock players backwards, dealing massive damage, and attack from afar by throwing a lump of debris or bashing a heavy object towards survivors; the Witch, a passive female infected, who, when provoked by loud sounds, lights, or proximity of survivors, will attack her provoker - she is able to incapacitate or kill the provoker (depending on the difficulty setting) in one hit. If her victims survive, she will attempt to kill them if not killed herself. In
Left 4 Dead 2
, the Witch will wander around in daytime levels.
Several new Special Infected are introduced in
Left 4 Dead 2
, all of which are playable in versus and the new Scavenge game mode. The Charger is an infected with a mutated, oversized arm, able to move quickly and knock players in its path off their feet. It can also grab players and smash them against the ground to deal major damage. The Spitter can project balls of mutated, acidic phlegm that splatter across an area, eroding the survivors' health as long as they remain within it. The Jockey can jump onto the back of a survivor and steer them towards other infected or terrain traps set by the Director. With the Jockey it is also a wise tactic in avoiding damage by posting up above the survivors and dropping down, with this you instantly gain control (if you actually landed down accurately) without having to short range jump to steer your survivor away.
Also there are other characters like the Boomer and The Hunter. These characters vary in their ability: Boomer - when you play with him in versus or Scavenge he is the weakest character because of 50% life and 0 armor and he's slow.
The Hunter in Left For Dead 2 is better because he jumps to the Survivors and causes a lot of damage to them with 250% life and it's the most difficult infected character that you can rid of.
Further advice if you are playing in Multiplayer mode-versus mode the infecteds should outnumber survivors such as:
4 humans
5 infecteds
or you can play 8 players 4-4.  Keep in mind the various differences in order to reach balanced play.