Lupin the 3rd is at it again! His latest endeavor involves obtaining the Trick Diamond, a priceless gemstone that was once part of Spain's Sagrada Familia cathedral. However, Lupin's plans are interrupted when he receives a challenge from his late rival thief, Mark Williams. In order to acquire the diamond, Lupin must return five stolen treasures to their rightful owners within a week. Failure to do so will result in him never finding the Trick Diamond. Despite the challenge, Lupin can't resist the opportunity for such a valuable prize. The situation becomes even more complicated when Lupin discovers that Mark's assistant, Fujiko, is hiding something from him. To make matters worse, the Lupin Gang must face off against the Mafia boss Ivan "Rats" Crockovich, who is determined to destroy the gang and take the diamond for himself.