Heroes and Demons

The story isn't the greatest, but the episode is the first time we see the Doctor anywhere but the sick bay, in a holodeck program based on the epic Beowulf. Some crew members who were in the same simulation have mysteriously disappeared and the Doctor is the best choice to go and investigate, as he is a pure energy being to begin with and cannot dematerialise. While investigating, he has an interesting set of adventures, including his first romance(!). Doctor episodes always tend to be interesting.