The human intestinal tract is a world inhabited by good bacteria, bad bacteria, and opportunistic bacteria, whose balanced numbers maintain peace. However, the host's deteriorating lifestyle allows the bad bacteria's leader, Boss, to launch an attack to dominate the intestines. The good bacteria's captain struggles against the bad bacteria corps due to insufficient numbers and abilities of postbiotics. A mysterious figure calling himself "Captain Jicha" then arrives in the intestines, summoning the postbiotic L-137 to aid in the fight.