Sao Maeshima, a member of the national idol group "Seventeen," secretly belongs to Earth's Defense Force, a mission unknown to the public. As the giant heroine "Sylphina," she plays a vital role in protecting Earth's peace. One day, while enjoying her rare day off, an emergency alert suddenly sounds. A massive spider-like monster named "Tarantis" has appeared, destroying the city and threatening Akihabara—the sacred ground of Seventeen. Sao immediately transforms into Sylphina and rushes to confront Tarantis at Akihabara Dome, the concert venue of Seventeen, to protect it from destruction. However, Tarantis uses its eight legs to tightly constrict Sylphina, gradually overpowering her with relentless, brutal force.