
The King — a cosmic deity of unimaginable power — gets drunk/high and accidentally obliterates most of the universe. After sobering up, he admits "it felt quite good", and goes on to say some wonderfully worrying things:
"We felt the beauty of all things, and felt love for all.
Did you see? We smiled a genuine smile.
Did you see? The stars splintering in perfect beauty.
Now there's nothing but darkness."
Earth is somehow the only celestial body that wasn't destroyed, and people are understandably quite upset with the whole situation. So the King coerces his son, the Prince, into collecting stuff from Earth in order create new stars and fill the universe with light again.
...
As the player, you take control of the Prince, rolling a katamari around the world in order to collect stuff. The controls are quite unique, utilizing both joysticks to give you full control of the rolling. You swoop up junk simply by rolling over it, and watch in awe as the ball keeps growing. It really feels like you're pushing this massive, uncontrollable thing around — and while it's not always easy, it's always a joy.
Stress-free and stressful at the same time, the game is all about exploring the vibrant world, and amassing things until the clump is large enough to be sent into space and form a star. The first goal is to reach a measly diameter of 10 centimeters, by rolling up various household objects like thumbtacks and hairpins, screws and batteries, caramels and cookies. But it's not large enough.
Soon, you'll have to reach a meter in diameter, with things like bug sprays, potted plants and stuffed animals.
Then ten meters, and it's okay if you roll up a few live animals at this point.
Then hundreds of meters, and you might start to wonder: When will this end? What will remain of this world?
...
"We want to roll everything in.
Don't stop, and let the love flow,
then we all can be,
together forever ~"

...
I think that true art is best experienced while forming your own interpretations, so please don't read the spoiler below if you haven't played Katamari yet!
I just wanted to mention that the creator, Keita Takahashi, has literally said that the Katamari series is a comment on rampant consumerism, and I find that really funny.
"I wanted to make more objects. If there are few objects, I feel lonely. If there are more objects, they will make things more colourful. But when they're rolled up, they're gone. I felt empty. I feel the same way about the disposable society. I think I successfully expressed my cynical stance towards the consumption society by making Katamari — but still I felt empty when the objects were gone."
...
"Oh how this blue planet,
spins so very peacefully,
but how the sadness...
It never seems to go away."