The player views Rocket Slime from an overhead view, and controls Rocket entirely with the face and shoulder buttons. This mode of play is based on the previous titles. Rocket has the ability to jump as well as hover, though his primary technique is the Elasto Blast, an attack that launches Rocket forward. The Elasto Blast's strength is determined by how long the player allows for it to charge. This technique is constantly used throughout the game, used to pick up objects and attack enemies. When an enemy or object is hit by the Elasto Blast, it is sent flying, requiring the player to move forward and grab the item or enemy before they land if they wish to pick them up. The player may hold up to three items and/or enemies at any given time. Any item or enemy may be used as a weapon against an enemy if thrown at them. Rocket has a health meter that consists of multiple hearts, where Rocket will lose half a heart if damaged. The player may increase it by finding a rare item.
Each stage has a certain number of Slimes, and a variety of different items and enemies that can be found within. The game uses a day/night system, causing differences in enemies. Enemies that can be found awake during the day are asleep at night, and vice versa. There are several differences in the levels, but one thing they have in common are the railways. These railways typically lead back to Boingburg, allowing the player to send items, enemies, and other Slimes back to Boingburg, or leave the level via the railways. When a Slime is sent back, the Slime will send a letter to the player, including either an item or a recipe for an item. When an enemy is sent back, that enemy will become a resident of Boingburg, though only one enemy per species is allowed. When an item is sent back, it can be used as ammunition for Rocket's tank or sold at a shop. Another common element is the tank battles, where the player may battle a Plob tank. Defeating a tank gives Rocket a specific item and a Slime, who returns back to Boingburg. Most stages have a boss battle near the end of the stage. All bosses have a health meter, which must be depleted for Rocket to progress any further. When the player leaves the stage, a screen is shown detailing what Slimes the player acquired in this session of the stage, as well as any items and enemies that he or she collected.
In between stages, the player is sent back to Boingburg to rest. In the beginning after the Plob attack, there are no Slimes remaining, and the Plobs have destroyed much of the town, leaving large spiked balls blocking much of the houses and the village. Most of these spiked balls have ST written on them, meaning "Slime Tons". The number listed is the number of Slimes the player must rescue in order to get rid of the spiked ball in question. After a certain point in the game, the rescued Slimes rebuild Boingburg back to its state in the prologue. There are several things the player may do while in Boingburg. Early on, the player only has the ability to save his or her game, but eventually gains access to much more, including tank customization, a shop, a tank arena, a surfing mini-game, and an area where the player may synthesize items using recipes that are collected throughout the game.