Mesal Gear Solid runs on the modified version of the main Ape Escape 3 engine that implements traditional elements from the Metal Gear series, such as multiple alert phases, clinging to walls, hanging onto ledges, and first-person shooting. The player controls Pipo Snake, a simian covert-ops agent who has been tasked to help out Solid Snake in thwarting a group of guerrilla apes who have hijacked the latest model of Mesal Gear (a simian version of Metal Gear).