The year is 2108, Earth remains under constant threat of attack from a race we have codenamed "Vectoids". We have been refining our abilities to defend ourselves, but the Vectoids have been changing their methods of attack to combat us. Your goal is to eliminate the Vectoid threat before they reach the end of the path. You do this by constructing towers along the path. Success is not based solely on the types of towers you select, but where you place them and what upgrades and bonus options you take.