The game takes place two years after the 2017 war of Monster Attack, with the Ravagers (known only as "Invaders" in the English translation) repelled after the destruction of their Mothership and the Gigantors thought to have been exterminated. The introductory cutscene states that the Earth Defense Force was established in the aftermath of the first war with the Ravagers, which rather contradicts Monster Attack saying it already existed before the first Ravager invasion. Reports come in of Gigantor sightings in the city of London, and what begins as a cleanup effort soon becomes a struggle for survival as massive numbers of Gigantors emerge across the world. A giant above-ground hive appears in Japan practically overnight and powerful Flying Ants emerge to attack the terrified population, with the EDF's initial assault on it proving futile. As the situation becomes more desperate, the EDF deploys its forces underground to track down and destroy the Queen and end the Gigantor threat once and for all, but they are interrupted by the sudden arrival of a Ravager fleet centred on another Mothership, the same kind that wiped out militaries across the world in the 2017 war.

The game takes place two years after the 2017 war of Monster Attack, with the Ravagers (known only as "Invaders" in the English translation) repelled after the destruction of their Mothership and the Gigantors thought to have been exterminated. The introductory cutscene states that the Earth Defense Force was established in the aftermath of the first war with the Ravagers, which rather contradicts Monster Attack saying it already existed before the first Ravager invasion. Reports come in of Gigantor sightings in the city of London, and what begins as a cleanup effort soon becomes a struggle for survival as massive numbers of Gigantors emerge across the world. A giant above-ground hive appears in Japan practically overnight and powerful Flying Ants emerge to attack the terrified population, with the EDF's initial assault on it proving futile. As the situation becomes more desperate, the EDF deploys its forces underground to track down and destroy the Queen and end the Gigantor threat once and for all, but they are interrupted by the sudden arrival of a Ravager fleet centred on another Mothership, the same kind that wiped out militaries across the world in the 2017 war.