14 September 1943: The legendary submarine Y1 "Katsonis" was sunk north of the island of Skiathos by the German submarine chaser UJ 2101. Through the book of XO Elias Tsoukalas who escaped capture and had to swim for nine hours to reach shore, secret documents, and crew members' diaries, the documentary unfolds the human stories woven around the submarine. Seventy-five years later, with the support of the Hellenic Navy, we search for the submarine sunk at 253 meters depth and film the wreck for the very first time.