‘Saru’ is Yataro Sarumaru (Hayato Ichihara), a talented young locksmith from a lower-town commercial district. He is, however, a complete fool when it comes to women. One day a girl who calls herself ‘Mayumi’ (Manami Higa) asks him to open the safe at a sports club, claiming she has amnesia and has forgotten its combination. Inside the safe is a briefcase full of money stolen the day before from a local bank. Knowing nothing about it, he takes the briefcase and finds himself with yakuza gangsters in hot pursuit. He has sworn to protect Mayumi, but now it appears that the police are after him, too. Who is this woman, anyway? What’s going to happen to him? Can he keep his promise?