Call of Duty 4: Modern Warfare
As opposed to earlier games in the Call of Duty series, the game features modern equipment and new features, many exclusive to the multiplayer part of the game, such as "killstreaks"; killing a number of enemies without the player dying in between kills allows access to various assets including airstrikes and helicopter support. A character can be positioned in one of three stances: standing, crouching, or prone; each affecting the character's rate of movement, accuracy, and stealth. Using cover helps the player avoid enemy fire or recover health after taking significant damage. As such, there are no armor or health power ups. When the character has taken damage, the edges of the screen glow red and the character's heartbeat increases. If the character stays out of fire, the character can recover. When the character is within the blast radius of a live grenade, a marker indicates the direction of the grenade, helping the player to either flee or toss it back to the enemy.
The player takes on the role of various characters during a single-player campaign. The characters' involvement in the plot occurs simultaneously and overlaps the events in the game. As such, the player's perspective changes from one character to another between missions. Each mission features a series of objectives; the player is led to each objective with the heads up display, which marks its direction and distance. Some objectives require that the player arrives at a checkpoint, while other objectives require the player to eliminate enemies in a specified location, stand their ground to defend an objective, or plant explosive charges on an enemy installation. After the credits, a special epilogue mission is unlocked for play, featuring a four-man squad retrieving a VIP from terrorists who have hijacked an airliner. The SAS rescue the VIP and escape before the plane is destroyed.
Call of Duty 4: Modern Warfare features team-based and deathmatch-based multiplayer modes on various maps. Each mode has an objective that requires unique strategies to complete. Players can call in UAV reconnaissance scans, air strikes, and attack helicopters, when they achieve three-, five-, and seven-enemy kill streaks respectively. A game ends when either a team or player has reached a predefined number of points, or the allotted time expires in which case the team or player with the most points wins. If the points are even when the time expires, Sudden Death mode is activated in which there is no re-spawning and the team who either has the last man standing, or achieves the objective first are the winners. If the player is in either of the two matches, then there is an Overtime match, in which the next team to win is rewarded the victory.
The player's performance in the multiplayer mode is tracked with experience points, which can be earned by killing opposing players, completing challenges, completing objectives, or by completing a round or match. As the player gains experience, they advance in level, unlocking new weapons, perks, challenges, and gameplay modes. The highest obtainable level is 55, but on the console versions of the game, the player has the option to enter "Prestige" mode, which returns their level to one and removes all accumulated unlockables. This process can be repeated up to 10 times with a different insignia being given each time.
Completing a challenge grants experience points and may unlock weapon attachments. As a player's level increases by gaining experience points within online games, it unlocks new weapons, perks, or challenges. As the player advances in levels, they earn the ability to customize their classes; this includes selecting their main weapon, side arm and special grenade type. Additionally, the player can select 3 perks, one from each of the three "Tiers", that can customize their character further. Perk effects include, but are not limited to, extra ammunition, increasing bullet damage by the player, or dropping a live grenade when the player is killed. The player is also given the choice to complete challenges in order to receive even more experience points; challenges include achieving a certain number of kills with a specific weapon, shooting down a helicopter or performing a number of head shots. Additionally, when the player attains a certain amount of headshots with a specific weapon, excluding sidearms, the player unlocks extra weapon "camos", or camouflage, to use for that specific weapon.