"The blue sea" is an action drama that tells the story of a mysterious young man who lost his memory. Jo Eun Hee is a female soldier in the navy who works with Sung Ki Soo to carry out special missions under UDU (Underwater Demolitions Unit) Sung Ki Soo is a navy lieutenant who is also a professor at the non-commissioned officer department who is the first to see through the identity of the young man who lost is memory.