The year is 2020, and 10 years have passed since the signing of the Interplanetary Treaty of Friendship between Earth's human race and the planet Leutz in the galaxy. During this time, many Leutzians have visited Earth, which has led to a variety of problems: illegal immigration, various crimes committed by some Leutzians, terrorism, and pets brought in by Leutzians going wild and turning into monsters that cause destruction and damage to cities. To address these issues, the Japanese government formed the special mission unit ASSAULT.