Flash must stop Captain Vulcan and his plan to take out all the American nuclear missiles after capturing the sub Trident; Flash battles sea creatures and is too late to rescue a sunken ship.
Flash and Iris ate at a lighthouse at Land's End and they are listening to the lighthouse keeper, Phil Anderson. He says he is maintaining the lighthouse so that his missing wife Phyllis can return to him "walking on the waters." The story then flashes back to several hours earlier when Barry Allen gets up to go to an urgent appointment. Iris wakes up and urges him to "pay a little attention...to your wife..." He runs through Gotham City on his way out to meet up with the destroyer USS Centaur. The Fastest Man Alive is late: the nuclear sub Trident left on its deepest test dive without him. Flash watches on the ship's scanners as the Trident makes its dive and things are going well until they lose contact with the sub.
Flash dives into the sea in pursuit of the sub, but along the way he encounters exploding sharks, an electrocuting octopus, and he is delayed and unable to find the missing sub. He returns to the ship with the bad news, and the Commanding Officer, and the news media on board, blame him for the submarine's loss, and the loss of the "irreplaceable crew and scientists." Flash is confronted upon his return by a large crowd, and he is slapped by a woman holding an infant because her husband was one of the ninety-nine people lost on board the Trident. Barry Allen and Iris are devastated by the loss, and his failure to rescue the sub. Unable to stand it any longer, Flash offers his help to the Navy to try and locate the missing sub, but his "request to help--DENIED!"
Barry and Iris decide to go and visit her friend Phil Anderson, the keeper of the lighthouse. Phil has heart problems and he recounts how he and his wife Phyllis met in the Army. Phil went to Viet Nam and he threw himself on a grenade. He survived his injuries and married Phyllis, with Iris acting as maid of honor. At some point after their marriage, the government asked them to go on a secret mission crossing the Atlantic in a sailboat under the guise of a "second honeymoon." The true purpose of their voyage is to "sail over the Greenland Shelf where enemy nuclear subs are believed lurking." They were captured by an enemy sub and attempted to escape. Slowly lethal gas is released into the compartment where they are hiding.
Only one of them could escape, so Phyllis insisted that Phil leave her behind. She promised she would return to him somehow. Flash decides to go looking for Phyllis. He goes to the Greenland Shelf and finds a metal door with interlocking steel teeth. He is captured by frogmen and blacks out, having failed again. Flash comes to and realizes that there is a secret sub base inside the shelf and sees the Trident. He is taken to see the skipper of the sub that captured Phil and Phyllis: Captain Vulcan. Imprisoned with Phyllis and the missing men of the Trident in a booby-trapped cell, Flash asks them if they are willing to risk their lives to try and escape. Phyllis, and the husband of the woman who slapped him, are quick to speak up and vote to try and break out.
Flash manages to break out of the holding cell and protect the crew from the explosion. He defeats the soldiers and disables their subs, and the Trident is able to leave. Phyllis waits for him because she is slowly dying from the gas she was exposed to on Captain Vulcan's sub. As Flash is desperately running as fast as he can back to the lighthouse carrying Phyllis, he sees Phyllis's spirit walking on the water toward the lighthouse and then he sees Phil's spirit walking out to join her. They meet, embrace, and wave goodbye to Flash. When Flash arrives, Iris is next to Phil's dead body. Flash puts Phyllis's body down next to him. Iris concludes by saying, "My heart tells me they're together again...for always!"