According to an old document found in the original Arsène Lupin's silk top hat, Lupin stole a beautiful statue, the Opal Venus, from the Palace of Elysees. He then left it with Gregoire, the second greatest thief after Lupin, for safekeeping. Now his grandson, Arsène Lupin III, decides to get back the treasure his grandfather left behind. He and his friends get as far as Gregoire's castle, but there is only one way to get across the garden which the Venus is decorating, and thus collect it.
—Tenchi