Having returned to Japan after an extended absence, Lupin challenges the Marukin line of safes, renowned for being absolutely unbreakable, and cracks them at every turn. Having completely lost confidence in himself, Marukin experiences the grief of bankruptcy. As far as Lupin is concerned, it's his natural ability at work, but Marukin's son, Tetsutaroo, challenges him in his father's place. Giving Lupin poison, he forces him to accept the challenge of opening the safe he give him - a safe which has neither lock nor combination, nor even a door!