Horror in the Underworld (1971)

Mi-Hye and Mi-Ran sisters get to inherit 100,000 dollars from their long lost father. The next day Mi-Hye's lover Baek Seung-Ji finds that Mi-Ran got killed, which makes him a suspect of the crime. To prove he is innocent, Seung-Ji investigates the murder. Soon it turns out that a band of North Korean agents including Dr. Kim, Mi-Ran and Mi-Hye planned to kidnap Seung-Ji who worked as a cipher officer in the military. The police arrest all the North Korean agents who were on the verge of kidnapping Seung-Ji.