Kazuto Tokino upholds his grandfather's legacy by operating the Tokino Bath House, persisting despite parental objections over its financial viability. He resides there with his sister Rika, who helps manage operations but shares their family's profitability concerns.

Princess Valkyrie's crashing spaceship fatally wounds Kazuto, prompting her to save him by transferring half her soul. This act de-ages Valkyrie into a child while reviving Kazuto, creating a soul-link that allows her temporary adult transformation through kisses during emotional synchronization. Kazuto becomes Valkyrie's protective guardian, curbing her mischief while developing romantic feelings for her true self. Their bond deepens, earning recognition from Valkyrie's family who eventually name him her official fiancé.

Kazuto navigates key relationships:
- **Akina Nanamura**, his childhood friend, harbors unrequited feelings he remains oblivious to. He maintains supportive friendship without reciprocation, focused solely on Valkyrie.
- **Sanada**, Valkyrie's maid, transitions from distrusting Kazuto to accepting him as Valkyrie's partner. She instigates conflicts like brainwashing girls into catgirl maids, requiring Kazuto's intervention.
- **Hydra**, Valkyrie's rival, antagonizes Kazuto while attempting Valkyrie's forced return to Valhalla. Kazuto thwarts these efforts to protect Valkyrie's choice to stay on Earth.

Later, Kazuto joins Valkyrie in a treasure-hunting ritual to secure wedding fortune. He steadies her through "wedding jitters" and remains devoted after a space pirate attack during their ceremony splits Valkyrie into separate adult and child entities. Kazuto adapts to caring for both forms, indefinitely postponing their marriage.