Star Trek: Voyager (1995)

S1E11 Heroes and Demons

Stardate: 48693.2. While immersed in his holo-novel "Beowulf," Harry Kim suddenly vanishes from the holodeck. The characters claim he was killed by a legendary creature called "Grendel." When both Chakotay and Tuvok also go missing, the Doctor is sent into the holodeck to uncover the mystery behind these strange disappearances.