Blue.

Two friends in the Korean Navy, Lee and Kim are both part of an elite diving squad, specialising in emergency deep sea salvage dives. Lee is straight-laced and takes his duties seriously, while Kim treats the Navy as a lark. When Kang, a diving instructor and Kim's former girlfriend, is posted to the unit, this creates tension between the friends as they compete for Kang's affections. The tension is heightened when Lee is promoted ahead of Kim, creating a rivalry between the two. Kim's gung-ho approach to diving, and the danger he poses for himself (and his fellow divers), leads to further problems. Matters come to a head when an incident at sea causes the sinking of a submarine, requiring the unit to attempt a dangerous salvage rescue of the sunken submarine.