Luna and the Sailor Senshi suspect that the treasure at an embassy party being held by the kingdom of D, the world's largest producers of diamonds could be the Silver Crystal they've been searching for. The Dark Kingdom want to find out if the crystal is the Silver Crystal too, so they can use it to revive their master.
Usagi disguises herself as a princess to sneak into the embassy party. When Ami and Rei discover Princess D's bodyguard is actually Nephrite, he has his shadow possesses Princess D so she can escape with the kingdom's treasure.
Nephrite regroups with Jadeite, Zoisite, and Kunzite, they introduce themselves to the Sailor's as the Shitennou. After Princess D discloses her treasure as a diamond carving of herself, Tuxedo Mask reveals to Luna that he is also searching for the Silver Crystal.