The player must navigate the protagonist, Solid Snake, through a nuclear weapons facility without being detected by enemies.[17] When Snake moves into an enemy's field of vision, he sets off an "alert mode" that draws enemies.[18] The player must then hide until "evasion mode" begins; when the counter reaches zero, the game returns to "infiltration mode", where enemies are no longer suspicious. The radar cannot be used in alert or evasion mode. In addition to the stealth gameplay, set-piece sequences entail firefights between the player and enemies.[19]