From Wikipedia
The game opens with the King of All Cosmos, his Queen, and the Prince, enjoying some tennis while on vacation. However, a powerful serve by the King causes the tennis ball to rip through the fabric of the universe, creating a black hole that sucks everything but the planet Earth into it. In order to correct it, the King commands the Prince to roll up katamaris on Earth to recreate the planets, sun, and other celestial features. Once the Prince completes this, the King then orders him to create a katamari large enough to plug up the black hole and restore order to the universe.