Rats! is a 2D platform video game designed for single-player where players play as the character Mr. Rez, described as "a radical yo-yo playing rat-for-hire assassin with an attitude and cool, black shades" who was an "awful assassin".
Rats! uses a password-based save system which allows players to resume their gameplay where they last left off by inputting the correct password for the level.
There are eighty-five levels included within the game. These levels are spread over five environments (themes/worlds) with each environment spanning fifteen levels and two bonus levels and having a time limit. Failure to complete a level within the allotted time will result in black devil enemies appearing.
Each of the environments has its own enemies and unique boss at the end of the level. Each level and boss becomes increasingly harder as the player progresses.
