Kaito Takagi is a background character in his own story: an ordinary high schooler earning pocket money by hunting slimes in the dungeons that have opened across Japan, while quietly admiring his childhood friend Haruka Kasuga from a distance. Everything changes when a golden slime he's never seen before drops a servant card worth a fortune. Summoning it calls forth Sylphy, a valkyrie who binds herself to his service. With a mythical ally under his command, Kaito starts climbing out of obscurity and into the front lines of dungeon exploration. This manga adapts the ongoing light novel.