Because of an alien bug biting people and transforming them into bug humanoid monsters most of the population on Earth got transformed in the process. Those that weren't bitten either got eventually transformed, resisted or became food to the insect people. You are the last human survivor in this doomed world. The blue marble became a brown marble thanks to those bugs. And those bug people are a few heads taller than an average human. Their skin is like an armor and the ladies are especially aggressive if not weirdly erotic. However your mission is to find a way out of here or die trying.