You play as Travis Touchdown, an otaku-turned-assassin who before the start of the game, killed the 11th best hitman with a beam katana he won on a online auction. You spend the game trying to kill the top 10 assassins above you to become the best.
The gameplay can be divided into two parts: the assassin missions and the process of gathering up the money needed for said missions. The latter is done by driving around Santa Destroy (Travis' town) on your super-powered motorcycle and finding missions to earn you cash for assassin fights, videos, clothes, etc. These mission range from oddjobs like lawn mowing and litter gathering, to hit jobs. Most of them are pretty entertaining for at least the first time or so, simply because they all utilize the motion sensor in one way or another.
The combat is simple, but enjoyable. It's great fun tearing through a group of enemies and making them explode into clouds of blood (or pixels, if you're not in America). The actual assassins are intriguing characters, and include the likes of evil superheroes, corrupt vice cops, and bat-wielding lolitas, and their battles are the highlight of the game.
I'm a massive fan of the game, and I recommend to you if like old-school fun and bloody, hyper-stylized ultraviolence.