The Blast Furnace is a 1950 North Korean film directed by Min Chongshik.
It was produced by the Division of Film Productions of the Korean Workers' Party.
Ryongsu and his assistant Hyeyoung work day and night researching fire bricks.
Ryongsu sends a letter to Ryongyeon to clear the air, but the misunderstanding remains, due to her inability to read.
Ryongsu's research is endangered by spies, and Ryongyeon appears and saves the day.