You're walking around inside a human body, where you'll have to repel the attacks of all kinds of viruses and bacteria. Shoot them, and then grab the bonuses that are released. You can earn all kinds of bonuses: recovery, health, additional attack power (damage), bombs and shields. You can also choose from all kinds of weapons to fight all the dangers.
Use the mouse to aim and to shoot. Use the arrow keys/WASD to move around.