Yuji Sano was a normal guy working a soul-sucking nine-to-five job when he suddenly found himself transported to a fantasy world where he restarted his life as a Monster Tamer. After befriending the Slimes, Yuji is quickly able to gain a second character class, ranking up his stats and learning new skills as he encounters one surprise after another.

In a forest infested with ghostlike monsters called wraiths, Yuji finds a facility of an organization known only as the "Laboratory." In order to uncover the truth of the organization, he sends his slimes to infiltrate, which they easily accomplish by way of their abilities. However, the organization seems to have knowledge of a "Tamer Yuji," which could spell disaster!