Harry Kim's holonovel based on the Old English poem Beowulf, in which crew members vanish until the Doctor enters it on his first away mission.
The Beowulf holonovel is a programme run by Harry Kim aboard Voyager, with the user as the hero who comes to the hall of King Hrothgar to face the monster Grendel. Its characters include the shieldmaiden Freya and the suspicious Unferth.
It is the setting of Heroes and Demons. Kim disappears while using it, and Chakotay and Tuvok vanish too when they go in to look for him. The programme cannot be shut down, so Janeway sends the Doctor, who as a hologram should be safe. It is his first mission outside sickbay, and he takes the name Schweitzer for the occasion.
Grendel proves to be a photonic lifeform, acting in answer to Voyager's capture of energy samples that were in fact living beings. The Doctor returns the samples and the missing officers are released, but Freya dies protecting him and he gives up the name he had chosen.