On the morning of 19 February 1917, German submarine UC-18 opened fire on Royal Navy Q-ship the Lady Olive. As the U-boat drew near to assess the damage it had inflicted, the Lady Olive counter-attacked. Both vessels were said to have sunk. For decades, divers and historians have searched to discover their fate and understand the story that led to this encounter. This documentary follows film-maker Karl Taylor and his specialist team over four years as they search to uncover what happened on that fateful day in 1917