The first of only two scripts of the original series to written by Doctor Who fans, the other being Ghostlight.
On first day of filming, the story's writer Andrew Smith visited the set. He suffered from a virus infection and was so unwell that he vomited on the Marshmen costumes.
Andrew Smith's original outline involved the TARDIS landing on the planet Alzarius, where monstrous Marshmen rise from the swamps during the time of Mistfall. As the Doctor and Romana explore, a space freighter crashlands on Alzarius, and the time travellers must help its crew repair the ship while fending off the Marshmen and gigantic cave-dwelling spiders. They are aided by a young Marshchild, who has been rejected by the rest of her kind due to her pacifist nature. In the end, the Marshchild sacrifices herself to keep the Doctor and Romana safe until the mists vanish and the Marshmen retreat to the swamps.
Adric is based after The Artful Dodger from the Charles Dicken's novel "Oliver Twist".
Andrew Forbes is amused by the fact that his character Omril has a name in the credits but his name never mentioned in the serial's dialog.