Portsmouth, 1803. The wedding of Marianne Morris and Jem Templeman is supposed to be a happy affair but Marianne soon learns that her husband is not the man she thought he was. When Jem runs away to sea in a drunken attempt to shirk his duties, Marianne decides to take decisive action. Rather than stay at home to be mocked as a deserted wife, she disguises herself in her brother's clothes and joins the Navy, determined to find Jem and bring him home. But Marianne is soon to learn that life at sea is hard and her voyage will surely be long and arduous as Nelson has orders to find the French fleet and has vowed not to set foot ashore until the battle is over.