The Doctor is sent to the holodeck to rescue a lost crew.
When Harry Kim goes missing on the holodeck during a program based on the epic poem Beowulf, the Doctor is sent to find him because he is the only crew member who can survive inside the simulation. The hologram has to leave sickbay for the first time.
The episode is the first to take the Doctor out of sickbay in any serious way, and it hints at his later development into a full member of the crew. It also puts the characters inside a mythic setting, and it reveals that the creatures inside the program are not what they appear to be.