Goemon encounters Nezumi-Koozoo IV(Ratboy Jirokichi IV) the fourth decedent of a great thief. He tells Goemon that their ancestors teamed up at the International Thieves' Championship. But they were defeated by Lupin's grandfather Arsene Lupin the 1st. Ratboy suggests Goemon that they should team up and avenge their ancestor's dishonor. And to do that they'll beat Lupin by stealing secret data from the Sakurada Police Station before Lupin gets the chance in nabbing it.
—J LeGault