The Treasure My Grandfather Left Behind

The Opal Venus, a statue which Lupin's grandfather, Arsène Lupin, stole from the Palace of Elysees. According to an old document found in Arsène Lupin's silk top hat, Lupin left it with Gregoire, the No. 2 great thief of the time. Lupin's feeling is, if my grandfather left it, I'll go get it back. So saying, he and his friends get as far as Gregoire's castle, but there is one way to get across the garden which the Venus is decorating, and thus collect it! What the hell is the gimmick involved?