The S.S. Queen of Glasgow is crossing the Atlantic in 1942, one day out from Liverpool on its way to New York. After losing its convoy, the Queen of Glasgow continues in the fog. On the deck, passenger Carl Lanser is looking out at the ocean when a steward comes out and informs him that he should go in if he wants to eat.
Lanser goes into the dining room and notices a young girl who has dropped her doll. He picks it up for her as one of the passengers, Jerry Potter, comes over and introduces himself. Potter explains that they have been looking for Lanser throughout the trip and invites his fellow passenger to join himself and two others for a coffee. Lanser hesitantly sits down at the table and Major Devereaux and his secretary, Miss Stanley, introduce themselves. They ask if he is heading away from home or toward home and Lanser says that he's heading away.
Captain Wilbur comes in and explains that he took supper on the bridge but wanted to meet with them. He admits that the fog is too thick for them to see their convoy and hopes to make contact in the morning. When Devereaux worries about German wolf packs of U-boats converging on them, Lanser speaks up and says that U-boats packs don't hunt single ships. Wilbur agrees with Lanser, warning that the threat lies in a single ship coming across them in the fog. Potter complains about the cowardice of a torpedo strike and Lanser again exhibits knowledge of U-boats, saying that a sub would surface and then shell them with its deck guns. Wilbur off-handedly comments that Lanser sounds like a U-boat captain and the passenger jumps, dropping his cup of coffee on his jacket.
As the steward offers to clean Lanser's jacket, Wilbur explains that the freighter wasn't intended for passengers. He asks Lanser where he comes from and the passenger says that he was in Frankfurt. However, he can't remember how long he was in England before boarding the ship for America. Lanser breaks off and say that he's not feeling well and goes to his cabin.
Later, Miss Stanley is going to her quarters when she finds Lanser on the deck. He asks if they have met before and she says that she hasn't, and Lanser explains that everyone aboard the Queen of Glasgow seems familiar to him even though he can't remember any of them. Miss Stanley sympathizes, saying that she's often felt a sense of deja vu, but Lanser says that it's more than that. He has no memory of boarding the ship: his first memory is of standing on the deck just before the steward called him in for supper. He does remember his name but can't remember what he was in, although it seems on the tip of his tongue. Miss Stanley suggests that he get some rest but Lanser says that he feels like he's in a waking nightmare and that he has a strong sense of foreboding. Looking out at the ocean, Lanser suddenly blurts out that there is a U-boat following them.
Before Miss Stanley can respond, the First Officer comes out and asks Lanser to meet with the captain. They go to the bridge and Wilbur asks Lanser for his passport, since his number isn't on the manifest. Lanser says that it's in his quarters and the First Officer says that they can get it later. Wilbur asks if there is anything Lanser wants to tell them and the man says that the only things he can remember his name and his place of birth. The captain notes that he knows about U-boats but Lanser doesn't know how he knows about them. Wilbur tells Lanser to go to bed and they'll talk in the morning. After the passenger leaves, Wilbur tells the First Officer to send a steward to get the passport from Lanser.
In his quarters, Lanser goes through his baggage and the steward arrives for the passport. As he locks through the cabinet, he sees a German naval captain's cap inside. Lanser snatches it away and says that he'll put it back himself. However, as he looks at it, Lanser is shocked to see that it has his name on it, identifying him as a German U-boat commander.
The ship continues through the fog and Wilbur orders the engineer, McCloud, to give them full speed for a full hour. The engineer says that he'll do what he can and Wilbur warns his First Officer that they need to catch up to the convoy or a U-boat will eventually find them.
Lanser goes to the bar and drinks, and comments to the bartender that the engines sound labored. He looks at the clock, which shows 12:05 a.m., and says that something will happen at 1:15 but he can't remember what. The First Officer comes in to get the evening tray for the bridge crew just as the engines stop. Lanser nervously asks why they've stopped and the First Officer assures him that it's routine maintenance. Much to the officer's surprise, Lanser blurts out that they're defenseless and adrift on the high seas. The first Officer doesn't see any cause for concern, assuring his passenger that the engines will soon start up, but Lanser says that they'll drift until 1:15.
McCloud doesn't get the engines going and at 1:15, Lanser says that they have to abandon ship. The bartender has disappeared and Lanser runs to the bridge. A U-boat spotlight homes in on him and Lanser sounds the alarm, running from cabin to cabin. No one responds and Lanser rounds a corner to see the passengers huddled in the corridor, all staring despairingly at him. He screams at them to get to the lifeboats but they disappear as he looks on. Lanser runs to the deck and grabs a pair of binoculars, and looks out into the night. He sees the sub and is shocked to see himself on the deck., wearing a German captain's uniform.
The captain gives his crew the order to shell the Queen of Glasgow. Time seems to start and the passengers and crew reappear. They die one by one as the shells rain down on the ship. Lanser falls over the side and plunges into the waters as the ship goes down.
Once the U-boat has finished its work, it submerges and Captain Carl Lanser goes to his cabin. Lt. Mueller comes in to make his report and Lanser notices that he's shaken. He demands to know what's wrong and Mueller admits that he has trouble reconciling the slaughter of innocent men and women without warning. Lanser points out that the enemy has no such trouble but Mueller persists, wonder if God will damn them as murderer. The captain says that he's a fool and a hopeless mystic, but Mueller says that he's had dreams of being sent to a hell where he's forced to relive the agony of their victims, night after night as punishment for their crimes.
Sometime later, Captain Carl Lanser, unaware of who he is, is aboard the S.S. Queen of Glasgow is crossing the Atlantic. A steward calls him to dinner as the cycle repeats... and repeats... and repeats for all eternity.