It gives a somewhat uncommon glimpse into Klingon mythology, most notably concerning Gre'Thor, the Klingon version of Hell, and the role of the Barge of the Dead. It was based on an early idea for the Star Trek: Deep Space Nine episode "Soldiers of the Empire."

It was never determined in the episode if B'Elanna's trip is real or simply her mind playing through the psychological implications of her childhood.

B'Elanna Torres leaves the ship to track down a long-range probe. On the way, her shuttlecraft encounters an ion storm. Torres performs a high-speed landing into the shuttle bay, assisted by a tractor pulse to slow her landing. Sustaining only minor injuries, B'Elanna is shown recovering in Sickbay after Tom Paris had examined her while she was still in the shuttle.

The crew discovers a strange piece of Klingon metal which was found when the shuttle crashed, presumably carried into Voyager after B'Elanna encountered it in the ion storm. When Chakotay brings the metal to her and they discover it came from the Klingon Empire, they exchange remarks about how the Empire may have been to the Delta Quadrant hundreds of years earlier. Harry Kim comes to B'Elanna's room later to discuss the metal as well and after he leaves, the metal suddenly starts to seep blood all over the table. When Torres examines it further, the blood disappears. The scene changes to B'Elanna and Ensign Kim scanning the device in the engineering section of the ship, where Kim remarks that their scans seem to agree that it is simply a piece of metal and that it is very late at night for them both to be up. Neelix, the ship's head chef and official morale officer, greets B'Elanna and reveals that he plans to throw a Klingon-themed party celebrating the find of the metal out in the Delta Quadrant.

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Wikipedia