After the destruction of their hometown, childhood friends Kafka Hibino and Mina Ashiro make a pact to become officers in the Defense Force—a militarized organization tasked with protecting Japan from colossal monsters known as "kaijuu." Decades later, the 32-year-old Kafka has all but given up on his dreams of heroism. Instead, he cleans up the remains of the slaughtered kaijuu after they are defeated by valiant soldiers—including Mina, who has successfully achieved their shared goal. Upon meeting his new coworker, Reno Ichikawa, Kafka faces a mirror of his past self: an ambitious young man whose one desire is to fight as a member of the Defense Force. Unfortunately, the two are soon involved in a freak encounter with a rogue kaijuu. Though Kafka demonstrates his innate heroic nature and rescues Reno from certain doom, he is left gravely injured. While both men recover in a hospital, Kafka is seemingly attacked by another one of the beasts. As a result, he gains the ability to transform into a humanoid kaijuu with the strength and powers of the massive monsters menacing Japan. Dubbed "Kaijuu No. 8" by the military, Kafka resolves to use his newfound gifts for the greater good. Tied together by mutual respect, Kafka and Reno set out to join warriors like Mina at the forefront of the Defense Force.

The year is around the 1920's and the darkness of the demons have arrived. Now, only the people with large amounts of spirit energy can save the Earth. The Flower Division is now going to be sent out destroy the demons for they're our only hope for survival with their new steam-powered machines and unique fighting techniques.

An unseen evil lurks in the streets of Paris. Unless something is done, the darkness will unleash its destruction and consume all that is good. The City of Love will be lost to her people forever. All hope rests within the hearts of five young ladies. The only problem is, they don't even know it yet. But will an apprenticed nun, a reserved aristocrat, a hardened criminal, a traveling circus emcee and a Japanese girl experiencing loss set aside their unique differences and come together as one for the defense of many. The Paris Fighting Troup is born.

Sakura travels to the capital with aspirations of defending the city from the demonic forces of the Black Sanctum Council like her father before her. However, things are not as she imagined as in addition to using her great spiritual energy to pilot a mech called a Kobu, she must also perform on stage as an actor as The Imperial Flower Division's cover is an art theater. Making a fool of herself and ruining a production gets her on everyone's bad side and somehow she must learn to work with them as well as prevent the enemy from destroying several shrines which protect the city.