An ancient ship comes out of deep space toward Babylon Five. An automated message identifies it as the USS Copernicus. In Downbelow, a raving human lurker looks out a porthole, sees the ship, and runs screaming. His name is Amis (Dwight Schultz) and he ends up in the Zocalo, shouting about armies of darkness and soldiers of the Devil. He accosts Mollari (Peter Jurasik) and G’Kar (Andreas Katsulas) before Chief Garibaldi (Jerry Doyle) takes him into custody. Captain Sheridan (Bruce Boxleitner) and Commander Ivanova (Claudia Christian) watch maintenance bots examine the ship, and Garibaldi joins them in the C&C.

It is a century-old sleeper ship, pre-dating jump gate technology, in which the crew and passengers were placed in cryostasis. Only one life sign is detected. The ship is brought in and the command team join Doctor Franklin (Richard Biggs) on board. One cryostasis tube contains a corpse, the other a living woman, who is rushed to Medlab. Garibaldi checks up on Amis in his cell. Garibaldi assumes he is experiencing flashbacks to the Earth-Minbari War. Ivanova stays aboard the Copernicus, trying to re-start its computers. She thinks she hears something.

Garibaldi lets Amis go, offering a counselor. Doctor Franklin revives the woman in Medlab. Ivanova finds no technical reason why the cryosleeper should have died, and suspects he was murdered. Franklin performs an autopsy and finds the reason why—several vital organs are missing. The living woman, now awake, is Mariah Cirrus (Anne-Marie Johnson) and says the other cryosleeper is her husband, Will Cirrus. Franklyn informs her that she has been in stasis for a hundred years and her husband is dead. She breaks down, blaming herself. Amis wanders into the docking bay and thinks he recognizes the Copernicus.

Mariah has a vision of herself in the tube with a strange and malevolent force looking at her. She blacks out. Franklin says her husband may have been murdered. Amis begins raving again. He tells Garibaldi that the Copernicus brings a soldier of darkness that he once faced during the War. In Downbelow, an alien lurker screams in terror and dies. His body is brought to Medlab, where he is discovered to have died in the same way as Will Cirrus.

Garibaldi relates to Franklin and Sheridan what Amis told him: He was one of 47 men stationed on a small moon during the War. All of the others were slaughtered. Garibaldi finds out that the Copernicus passed close to that moon. Franklin insists Mariah was in his quarters all night and could not be responsible for the recent death, but Sheridan puts a 24-hour watch on her. Word of the attack has gotten out and the Advisory Council wants a meeting. Delenn and Kosh are absent. The Markab Ambassador (Kim Strauss) insists that a soldier of darkness is on the station. These are agents of a great darkness long ago. Sheridan says he will get to the bottom of it all.

Garibaldi wakes up in the middle of the night, gets Amis to take him to Downbelow. Amis cannot find the monster but reveals that it let him live and fed on him. He runs off. Franklin finds Mariah on the Copernicus. She believes something was in the cryotube feeding on her. Garibaldi approaches Mariah in Medlab, asks if she can help him find the monster. She says she can feel its presence. Franklin joins them. A report of weapons-fire in Brown Sector draws Sheridan and a tactical team. They find Franklin treating an injured guard, attacked by the creature. Garibaldi has gone after it.

The tactical team finds Garibaldi cornering the creature, which has attacked Amis. It is nearly invisible and unharmed by gunfire. But a massive crossfire takes it out. Amis is treated for injuries. Mariah drops by and tells Franklin she is returning to Earth. Ivanova reports that the ship was heading for Z’ha’dum until it was drawn to Babylon Five. In his quarters, G’Kar opens the book of G’Quan and finds an illustration of the Soldier of Darkness.

Dwight Schultz has always played people not quite right in the head, from Howling Mad Murdock of the A-Team to Lieutenant Barclay on Star Trek: The Next Generation and Voyager. The creature, when you see it, if see is the right word, owes a lot to the Monster from the Id in Forbidden Planet. The name Amis may have originated in Amos, the Old Testament Prophet. Mollari’s clothing is becoming darker, like his soul. G’Kar’s comment that the future is not what it used to be comes from Yogi Berra. Earth obtained jump gate technology from the Centauri in the 22nd Century. The episode was generally liked and praised as creepy, but Doctor Franklin’s attraction to his patient, who had just lost her husband, was considered a bit creepy too and thought inconsistent with his personality. Still, the story is dark and foreboding, Dwight Schultz is riveting in a rare serious role, and his chemistry with Jerry Doyle is a nice touch.