fire for a few seconds, but it may not necessarily refill completely depending on how much damage the player took. Players can only carry a limited number of weapons since the inventory features a 4-slot system in which single hand-held pistols usually take a weapon slot, whereas heavy weapons like sniper rifles or rocket launchers can take up to three weapon slots. The player's movement speed is also altered by the weight of the weapon the player is currently holding. Besides the primary function, all of the weapons in Perfect Dark Zero have one or two additional function modes that generally grant the player with special abilities. For example, the Shockwave Rifle features an X-ray function which allows the player to see enemies through walls, while the Plasma Rifle has a cloak function that renders the player character invisible to enemies at the cost of its battery charge. Some weapons also feature more unorthodox secondary functions. For instance, the Laptop Gun can be deployed as a sentry gun, while the SuperDragon assault rifle can launch bouncing grenades. Campaign The campaign is divided into 14 missions in which the player plays as Joanna Dark. Each mission has a number of objectives that the player must complete in order to progress. To advance to the next mission, the player must complete all primary objectives, and if Joanna is killed or fails an objective, the player has to start the mission again. Missions also feature optional objectives that are not critical but add to the player's overall completion score. Some objectives require the player to use numerous high-tech gadgets. For example, a device called Datathief allows the player to hack into electronic devices, while another called Loctopus can be used to open locked doors. Stealth is an important element of the gameplay, as the player is often given the freedom to kill enemies without being detected by sneaking up behind them. Unlike the original Perfect Dark, every mission contains a single checkpoint at which the player may restart if Joanna is killed or loses beyond that checkpoint. However, checkpoints are progressively removed at higher difficulty settings. There are four difficulty settings on which a mission can be played: Agent, Secret Agent, Perfect Agent, and an extra one called Dark Agent, which becomes available once the player completes the entire game on Perfect Agent. As the player plays on higher difficulties, the game adds