We finally know what exactly it is you're trying to do in the 3DS Dragon Quest Heroes Rocket Slime sequel, known in Japanese as Slime Mori Mori Dragon Quest 3.
Square Enix detailed the game's story today. Things kick off when the Slime Kingdom's great treasures, the "Niji Orbs," are stolen by the Tails Brigade. There are seven orbs in all, and you set out to the seas on the Slime Ship to get them back. (I believe Slime Kingdom is translated as "Slimenia" and Tail Brigade as "Plob" in the English versions of these games)
Here you are vowing with great conviction to get the orbs back.
Joining the story details, Square Enix provided a look inside the ship that you use to navigate the seas. You find some screenshots below. The heart-shaped thing is your ship's engine, and you have to protect it if enemies manage to board your ship.
Additional Images (Total 14 Images, Use click action to expand)