The King of All Cosmos, a supreme, immense, omnipotent, flamboyant cosmic being bigger than Earth itself, got a bit too drunk and destroyed the cosmos in the process. The sky is now empty, which is a bit of a problem, as anyone could guess.
You play as the Prince of All Cosmos, a miniscule being; exponentially smaller than their father, the King; whose voice is too small to be heard but whose determination and perserverence are unmatched.
The King, after sobering up, realises that destroying all the stars in the sky is a bit of an issue, so he sends you, his son, to Roll The Katamari. By rolling the Katamari, The Prince (you) will slowly contribute to restoring the night sky around the Earth by rolling up the things that are on earth to make the stars.