Original: The player controls Cryptosporidium 137 (an obvious reference to the protozoan parasite Cryptosporidium), a member of the Furon race of aliens, who has come to Earth to harvest DNA from humans to continue the cloning process of his species.
Simplified: The player controls Cryptosporidium 137, a member of the Furon race of aliens, who has come to Earth to harvest DNA from humans to continue the cloning process of his species.