In October LP2, Rance has spent most of his money on women. His household is in debt, with taxes and bills waiting. Sill convinces him to get a job from the Keith Guild. Rance doesn't want to see Sill in danger like she was in Rance IV; so he tells her to stay home, and brings Athena 2.0 as his companion instead.

They're assigned a job at Happiness Pharmaceutical, getting information from the its President. Two weeks ago, a cave opened underneath the factory, with monsters emerging and attacking workers. The building's security guards set up a barricade, and are offering 150,000 gold to anyone who can stop the attacks. Rance and Athena 2.0 are the ninth adventuring party who've accepted the job; only four of the previous teams are still alive and active at the game's beginning. Each has room and boarding, but only the team who solves the case will receive any money.

Rance explores the main office, dormitories, and underground, discovering that the cave has two floors. He has to gather clues from staff members, such as security guards Conan and Kontama, scientists Rose and Joseph, and the president's daughter Eline; along with encountering rival adventurers like the easygoing monk Genri, the familiar Bird Lithfie and his new partner Kisara Copri (who Rance swears on having himself), and the demure Arnie Rolento; all while the mysterious Pink Mask helps him and Athena narrowly escape danger.

Travelling deeper, and looking for a way to overcome the dangerous monster gal Summon-chan, the adventurers find themselves all suspects when one among them is murdered. Rance and Athena 2.0 must clear the factory, clear their names, discover the truth about the attackers' accusations that Happiness Pharmaceutical is evil, and seduce the women nearby before Genri or Bird can make a move.