The seafood you sold in the fish shop attracts flies nearby. Now more and more flies are coming. They will fall on your seafood and contaminate them. Destroying them is the only way to keep your seafood fresh.
You will become a fly killer, using a fly swatter and a spray of insecticide to eliminate flies. To reach a high score, you must make as many combo hits as possible. And when you kill enough number of flies, you will unlock the ultimate weapons which are much more powerful. Be careful with the green flies, because killing them is not good for you.