The world's largest uncut diamond has been discovered in South Africa, and naturally Lupin goes there and gets it. Unable to cut the ore, no matter how he tries, he finally gets Goemon to do it. But this is part of the plan of the diamond concern, who have the cut diamond recovered by Guiness, the Chameleon Man. Guiness's special tank cannot be affected by ordinary bullets. This will require a round made from a diamond.
