You are Bubbles, a homicidal goldfish, and your objective is to kill every single fish in your fish tank. And you're gonna kill those fish with your lethal bubbles. Now that's fun and entertaining... but it's also challenging, because some of the fish in the tank aren't satisfied with simply being your next victim and won't go without a fight. They have harmful bubbles as well. So you'll have to avoid getting hurt.
You can replenish your health with fish food that falls from the top of the tank and looks like a lovely heart. Bubbles can also pick-up methamphetamines that allow Bubbles to swim unnoticeably faster for 30 seconds.
Of course a homicidal goldfish wouldn't be a proper homicidal goldfish if it didn't keep points for the poor fish it kills. Bubbles had better kill as many fish in the tank as it can before the boss shows up. He's a lot harder to kill than the other fish in the tank, and his bubbles are much more harmful.