Modern Warfare 2 has received highly positive reviews from various gaming websites, attaining a 94% aggregate score on Metacritic, with praise stemming primarily from its in-depth multiplayer component. Upon its release, the game sold approximately 4.7 million copies worldwide in 24 hours.

Campaign

Similar to previous installments within the Call of Duty series, the player assumes the role of various characters during the single-player campaign, changing perspectives throughout the progression of the story.

Each mission features a series of objectives that are displayed on the HUD, which marks the direction and distance towards and from such objectives. Tasks vary in their requirements, having the player arrive at a particular checkpoint, eliminate enemies in a specified location, stand his/her ground to defend an objective, or plant explosive charges on an enemy installation. The player is accompanied by friendly troops who cannot be issued orders. Laptop computers that contain enemy intelligence appear throughout the campaign and may be collected to unlock several game bonuses.

Cooperative

Modern Warfare 2 features a cooperative mode entitled Special Ops, which consists of independent missions similar in design to the "Mile High Club" epilogue mission from Call of Duty 4. These missions take place in a variety of locations from the campaign mode, but are not related to story of the campaign itself. Most of the Special Ops missions may be played alone, but all of them support two player cooperative play in which two players can play together locally or online. Such mode includes scenarios such as a snowmobile race between the two players, one player providing air support from an AC-130 with the other player conducting operations on the ground, capturing an enemy compound, and searching areas for and eliminating 40 or more enemies at a time.

Special Ops is divided into five separate groups of missions: Alpha, Bravo, Charlie, Delta, and Echo. Each mission may be played at three difficulty levels: regular, hardened, or veteran. Successfully completing a mission earns the players stars; there are up to three stars available for each mission: one star for completing the mission on regular difficulty, two for hardened, and three for veteran. Once enough stars are earned, more missions are unlocked.There are a total of 23 missions with their respective 69 stars available for unlock. At the end of each mission, a statistics screen displays how long the player(s) took in order to complete the level and how many kills were scored by each player, amongst other data.

Multiplayer

Gameplay in the online multiplayer mode. The weapon pictured contains the heartbeat sensor attachment.
The online multiplayer mode of Modern Warfare 2 retains the same experience points and unlockable reward system as that of Call of Duty 4, with game modes that include Free-For-All, Search & Destroy, Demolition, Domination, Team Deathmatch, and Capture The Flag.However, Modern Warfare 2 introduces several new features. Aside from new weapons, equipment, and perks that upgrade to "pro versions" after meeting utilization requirements, 15 different kill streak rewards can be unlocked and selected by the player. Among these is the ability to receive a supply drop after achieving four kills in a row, order a Predator missile strike after five kills, and call in an AC-130 gunship to strike from the sky after 11 kills.Another new feature is the in-game host migration; if a match host leaves the game, the current game no longer ends (as is the case in Call of Duty 4), as a 'host migration delay' allows a new host to be selected and the game to continue on.There is also the addition of an optional third person mode which can be used in certain game types.

In the Xbox 360 version of the game, the party chat system is disabled while the player is playing online in certain playlist types. This decision, designed to promote cooperation amongst team members, has caused some controversy within the Xbox Live community.

The offline (splitscreen and LAN) multiplayer mode still retains the experience and rewards system found online, a first for the series since the system was introduced. However, the rewards are separate from those that may be earned whilst playing online.

For the PC version, Infinity Ward has decided to implement a new matchmaking service: IWNET working through Steam. This system is nearly identical to the console version of IWNET. Dedicated server support is removed, eliminating the ability for mods or user-created maps to be incorporated. This removal has created anger among many PC gamers. Since the multiplayer aspect runs within Steamworks, the PunkBuster anti-cheat system utilized in previous titles has been replaced by Valve Anti-Cheat.In addition, the PC version shares the same 18-player cap as the console versions (matches are a maximum of 9 versus 9).

Characters

During the single-player campaign, the player controls five different characters from a first-person perspective. For the majority of the game, the player assumes the role of Sergeant Gary "Roach" Sanderson, a member of an elite, multi-national commando unit known as Task Force 141.However, the player begins the game through the eyes of Private First Class Joseph Allen, a Ranger stationed in Afghanistan, who later goes undercover in Russia for the CIA under the alias of "Alexei Borodin." Private James Ramirez, a member of 1st Battalion 75th Ranger Regiment stationed in the United States, serves as the played character during the defense of the United States eastern coast against a Russian invasion. John "Soap" MacTavish returns as a non-playable character for most of the game, but once again becomes the played character in the final three missions. In the five years since Call of Duty 4 took place, he has been promoted to the rank of captain in the Special Air Service. He now commands most of Task Force 141 and its field operations, acting as the superior officer of Sanderson.The player also briefly assumes the role of an unnamed International Space Station astronaut during an EVA in the minute or so prior to the station's destruction.

Several non-playable characters play prominent roles in the story. As mentioned, Captain John "Soap" MacTavish (voiced by Kevin McKidd) returns as a NPC for a majority of the game and serves as Roach's superior officer and mentor (similar to Captain Price serving as MacTavish's mentor in Call of Duty 4). Captain John Price (voiced by Billy Murray) also returns from Call of Duty 4 to assist Task Force 141 after MacTavish, Roach, and other members rescue him from a Russian Gulag. The other primary member of the unit is the mysterious Simon "Ghost" Riley, (voiced by Craig Fairbrass, who also voiced Gaz in Call of Duty 4)who conceals his face with a skull print balaclava.Sergeant Foley (voiced by Keith David), initially the squad leader of Joseph Allen while stationed in Afghanistan, becomes the squad leader of James Ramirez in the United States later in the story. Nikolai, the Russian informant from Call of Duty 4, returns to aid Task Force 141 at various points in the story. Lieutenant General Shepherd (voiced by Lance Henriksen) is the commander of both Task Force 141 and the Army Rangers, and becomes an antagonist after he betrays the Task Force members near the end of the game. Vladimir Makarov, a former Protégé of Imran Zakhaev (the chief antagonist from Call of Duty 4), serves as the primary antagonist throughout most of the story.Makarov is not killed during the game.

Plot

Modern Warfare 2 is set five years after the conclusion of Call of Duty 4. Despite the efforts of Marine Force Recon and the SAS, the Ultranationalists ultimately seize control of Russia and declare Imran Zakhaev a hero and martyr, erecting a statue of him in the heart of Red Square. Meanwhile, Vladimir Makarov, one of Zakhaev's former lieutenants, begins a campaign against Europe by committing numerous acts of terrorism.

The game begins in Afghanistan, where U.S. Army Ranger Private First Class Joseph Allen helps in the taking of a city from insurgents. Impressed by Allen's combat abilities, Lieutenant General Shepherd recruits him into Task Force 141, an elite, multi-national counter-terrorism unit under Shepherd's command. Meanwhile, two other members of Task Force 141, Captain John "Soap" MacTavish and Sergeant Gary "Roach" Sanderson, scale a mountain of the Tian Shan to infiltrate an airbase in Kazakhstan to recover a lost ACS module from a downed satellite. Allen is later sent on an undercover mission in Russia for the CIA under the alias of "Alexei Borodin." Allen joins Makarov in a massacre of civilians at the Zakhaev International Airport in Moscow. It is soon revealed at the conclusion of the attack that Makarov is aware of Allen's true identity and before leaving, kills him to expose his identity as an American, leading the Russian police to believe that America was responsible for the attack.
Riding a snowmobile in "Cliffhanger."

Angered by what was believed to be an American-supported terrorist attack, Russia retaliates with a massive surprise attack on the United States after bypassing its early warning system, revealing that the ACS module MacTavish and Sanderson recovered had already been compromised before its retrieval. Sergeant Foley leads his squad of U.S. Army Rangers, including Private James Ramirez, in the defense of a suburb in northeastern Virginia against the Russian attack. They then proceed towards a war-torn Washington, D.C., where more U.S. forces are fighting a desperate battle against the Russians for control of the capital city.

Meanwhile, Task Force 141 begins its search for evidence that would implicate Makarov as the true mastermind behind the airport massacre. Intelligence leads it to Rio de Janeiro, where the Task Force investigates leads on Makarov's contact, weapons dealer Alejandro Rojas. It finds out that a major nemesis of Makarov is locked up in a Russian gulag east of Petropavlovsk on the Kamchatka Peninsula. As a result, Task Force 141 assaults the prison and manages to free the prisoner, who turns out to be Captain Price. Price agrees to aid in the tracking down of Makarov. Price however, temporarily goes rogue, and leads Task Force 141 to raid a Russian port and gain control of a nuclear submarine. Price uses the submarine to launch a SLBM toward Washington D.C.. However, he sets the warhead to detonate in the upper atmosphere, which destroys the International Space Station, but spares the capital city from utter destruction. The resulting electromagnetic pulse cripples the vehicles and electronic equipment of both the U.S. and Russian forces in the city. Back on the ground, Ramirez and his squad, after avoiding the aircraft falling from the sky, head to the White House. There, they receive a transmission informing them that the Air Force is preparing to launch an airstrike against Washington D.C. unless they see green flares on the rooftop of the White House and other important buildings that are currently under Russian control. Ramirez and his squad fight their way to the roof of the White House and manage to set off flares in the nick of time, averting an airstrike. Green flares are lit on the rooftops of other D.C. landmarks, signifying that the city is in American hands.

Narrowing down Makarov's hiding place to two separate locations, Task Force 141 decides to split up. Captains Price and MacTavish travel to an aircraft boneyard in Afghanistan, while Sanderson and Ghost raid Makarov's safehouse in the Caucasus Mountains. At the safehouse, Sanderson and his team manage to obtain vital intelligence from enemy equipment. However, as Sanderson and Ghost reach the extraction point, Shepherd betrays and kills them, taking the intelligence with him to make him seem responsible for its acquisition. Meanwhile, Price and MacTavish quickly learn of Shepherd's betrayal and escape an ambush with the help of Nikolai. After managing to contact Makarov and learning of Shepherd's location, Price and MacTavish decide to take revenge on Shepherd in one final, bloody suicide mission. During the infiltration, Shepherd tries to escape and a long boat pursuit ensues.

Shepherd appears to have escaped when he drives his boat into the back of a hovering MH-53 helicopter. However, Price manages to shoot the helicopter down just before the boat he and MacTavish are on tumbles over a waterfall. After recovering from the fall, a heavily-injured MacTavish attempts to kill Shepherd with his knife, but Shepherd blocks the attack and in turn stabs MacTavish. As MacTavish lies on the ground, Shepherd prepares to execute him with his revolver, but before he can, Price attacks him and knocks the revolver away. Price and Shepherd then engage in an extended fist fight while MacTavish struggles to pull the knife out of his own chest. Finally managing to do so, he throws the knife at Shepherd, killing him. Price attends to MacTavish's wounds as Nikolai arrives in a helicopter to extract them. Nikolai warns the two that they will be pursued, but Price insists that MacTavish receive medical attention; Nikolai mentions that he knows a safe place to go to.

Game engine

The game utilizes the in-house IW 4.0 game engine, which is claimed to be a generation beyond the capabilities of the engine used in Call of Duty 4. Although proprietary, it is based on an unspecified id Tech engine, and can accommodate larger worlds, enhanced graphic detail, and more efficient rendering.

Infinity Ward has addressed the issue of enemies that continually respawn at different points of a level. The developer demonstrated that the game engine uses a "dynamic AI," which has replaced the infinite respawn system and allows enemies to act more independently. These "smarter" enemies are designed to actively seek out and drive the player forward through a level, and can break away from set behaviors such as following a designated route in order to attack. The player cannot depend on enemies to be found in the same locations as a previous play-through because enemies will behave differently each time a level is played.