Rocket Slime is set in the kingdom of Slimenia, a once peaceful land suddenly invaded by the Plob, a mysterious group of monsters identified in rank by an increasing number of tails, who has kidnapped all the slimes in the capital city of Boingburg. The player assumes the role of Rocket, the only slime left in the area, for being mistaken for a worm after swallowing a flute, who must embark on a long journey through the kingdom to foil the Plob's plot. In the process, the player must also rescue a cast of 100 slimes and call aide to select heroes, each with their own AI to assist in heated tank battles.