Call of Duty 4: Modern Warfare differs from previous installments of the Call of Duty series. Previous Call of Duty games have a distinct three country-specific campaign style, while Call of Duty 4 has a more film-like plot with interlaced story lines from the perspectives of Sgt. Paul Jackson of the Marines 1st Force Recon and Sgt. 'Soap' MacTavish of the British 22nd SAS Regiment. There is also a variety of missions where players control other characters, such as an unidentified AC-130U Spectre gunship operator and Lieutenant John Price in a flashback assassination mission set in Pripyat, Ukraine. The move to modern warfare has introduced a variety of conventional weapons and technologies to the Call of Duty series, such as assault rifles, night vision, .50 caliber sniper rifles, and C4 plastic explosives.
In 2011, Khaled Al-Asad begins a coup in the Middle East, and Russia is in the midst of a civil war between the government and Ultranationalists. Meanwhile Gaz informs Captain John Price, leader of Bravo Team, that a new recruit to the SAS, Sergeant John "Soap" MacTavish, is joining their squad.
Soap arrives at the SAS training facility in Credenhill, U.K, where he has a quick weapons training session with Gaz, meets Captain Price and the unit, and takes the CQB test to complete his initiation.
Bravo Team then proceed to the Bering Strait, to look for a suspected nuclear package on an Estonian freighter. After taking out the security detail the team find the package, but fire from enemy fast movers means they barely escape with the shipping manifest, which points to Al-Asad as the owner of the package.
To complete his revolution, Al-Asad executes the President of an unnamed Middle Eastern country, Yasir Al-Fulani.
Shortly after this, Soap, Gaz and Price rescue the Russian informant who supplied the intel on the cargoship operation, codename Nikolai, with the help of one of Price and Gaz's old friends, Sergeant Kamarov, and his Russian Loyalists.
While on their way out, the team's helicopter, Hammer 2-6, is shot down and they have to make their way across the fields, trying to evade enemies searching for survivors but eventually openly engaging them. After reaching a barn, Soap takes a FIM-92 Stinger and shoots down the chopper chasing them. An AC-130H Spectre then arrives to support the SAS to the extraction point, first while they're on foot and later in civilian vehicles. After they reach the junkyard, friendly birds come in to get them.
Meanwhile, Al-Fulani's death has prompted 1st Force Recon to invade the Middle East to search for Al-Asad. A team consisting of Lieutenant Vasquez, Staff Sergeant Griggs, Sergeant Paul Jackson and others infiltrate a small town and clear a target building and later a TV station where they think Al-Asad is broadcasting propaganda, only to find that there's no sign of him and the broadcast is on a loop.
The Marines then head to rescue a downed M1A2 Abrams, callsign War Pig (tank), from a bog, but heavy enemy fire prompts Jackson to destroy a ZPU so air support can come in to mop up. The unit then form up defensive positions on the tank as engineers come in to fix it, and later escort it back to the highway. They clear the forward area allowing War Pig to move up, and let the tank take some of the big targets. During this, Vasquez tells Griggs and his team to hold an area while he and Jackson complete the mission. At the end, they rendezvous with a CH-46 Sea Knight for evac.
At the climax of the invasion, the squad head to the capital city where they think Al-Asad has withdrawn to. After rescuing a pinned down advanced recon team, Vasuqez is informed that Al-Asad has a nuclear warhead in the area. They hasten to evacuate but the team's escort is shot down and they go back to save her. Having spent their time doing this, they do not escape in time and the nuclear bomb goes off, killing civilians, OpFor troops and 30,000 US servicemen, including Vasquez and Jackson. Griggs manages to escape, as he was not with the team during this.
At this point, Nikolai tells Price that Al-Asad may be at his safehouse in Azerbaijan. The SAS then head there, clearing several buildings in the village before finding Al-Asad. Price discovers the nuclear package didn't belong to him when Al-Asad gets a call from the Russian Ultranationalist leader, Imran Zakhaev. Price then executes Al-Asad.
Price then tells the team of when, in 1996, he was a Lieutenant assigned under the command of Captain MacMillan, on a mission to assassinate Zakhaev, an Ultranationalist who was trading nuclear fuel rods for weapons. The duo make their way through the ghost town of Pripyat, Ukraine, in ghilly suits and using minimal engagement to stay undetected. Eventually they reach a hotel where they stay on the top floor for 3 days, until Zakhaev arrives.
When the meeting is underway, Price uses a Barrett .50cal to snipe Zakhaev, blowing off his arm. The two then make their escape under pursuit from enemy helicopters, thinking Zakhaev will die from shock and blood loss. While oon their way to the extraction point, they take down a helicopter but it comes crashing towards them and injures MacMillan's leg. Price then carries him the rest of the way and they hold their ground, waiting for exfil.
Back in the present day, eight hours after the death of Al-Asad, the SAS use charges, a crashed Blackhawk minigun, and later a FGM-148 Javelin, to push back enemy forces until Griggs and the American Marines arrive in the Sea Knight "Gryphon Two-Seven."
With their target now shifted to Zakhaev, the actual owner of the WMD that killed the Marines in the Middle East, the SAS, USMC and Russian Loyalists perform a joint operation to try and capture his son, Victor Zakhaev, for information on his father's whereabouts. After a lengthy chase, the team corner him on the roof of a five-storey building and Soap moves in to restrain him, however he shoots himself to evade capture.
Angered by his son's death, Zakhaev takes control of an ICBM launch facility in the Atlay Mountains and threatens to launch nukes at the US in retaliation. Price then leads a joint British-American operation to stop this, but Griggs goes off course during the HALO and is captured. Soap, Gaz and Price then divert their course to rescue him before killing the power to the launch facility to allow Force Recon to breach the perimeter. Shortly after getting inside and linking up with Sniper Team Two, they witness two nuclear missiles being launched towards the US, with an estimated 41,000,000 casualties. The team therefore hasten inside and after taking out enemy armour with C4, cut the wire to the vents and get inside. After reaching the control room, Soap uses the abort codes sent by Baseplate, the team's command, to destroy the missiles in flight before they reach the Eastern Seaboard. Soap, Griggs and Price then rendezvous with Gaz at the vehicle depot, where they make their escape using Russian trucks.
With enemy vehicles and a Mi-24 Hind behind them, Soap, Gaz, Price, Griggs and some others head down the Russian motorway, but a bridge critical to their escape is destroyed and they have to suppress the enemy from behind cover. However, an oil tanker soon explodes, incapacitating everyone but Griggs, who tries to drag Soap to safety but is killed in the process by Zakhaev's guards and the man himself executes Gaz. A wounded Price slides Soap his M1911, with which the latter dramatically kills Zakhaev. Kamarov and the loyalists soon arrive, where Soap is carried away on a stretcher and a medic begins to resuscitate Price, but the fate of the two is unknown. Meanwhile, a news reporter announces a series of nuclear missile tests in Russia, and that the search for a ship in the Bering Strait has been called off, showing the public do not know what has actually happened in the six-day crisis.