The story sets up a mildly antagonistic relationship between the various incarnations of the Doctor, for humorous effect. The Second and Third Doctors bicker, compete, and try to put each other down. Even the First Doctor dismisses the others by saying, "So you're my replacements - a dandy and a clown!" This kind of relationship between the Doctor's selves was seen again in subsequent multi-Doctor stories. Patrick Troughton and Jon Pertwee in particular enjoyed the banter so much that they carried on the mock competition whenever they appeared together at science fiction conventions.
Early scripts concerned the 3 Doctors joining forces to combat various incarnations of Evil, including zombies and the goddess Kali, though this was modified after the poor health of William Hartnell prevented him from playing a more active role. Other suggestions included a role for Frazer Hines reprising Jamie McCrimmon (which was abandoned as he had prior commitments to Emmerdale (1972)) and a possible cameo for Wendy Padbury as Zoe Herriot.
Jo Grant interprets the Doctor's explanation of his earlier self, "I am he and he is me", as a misquote of a lyric from the Beatles song "I Am the Walrus". Other lyrics from that song are included in her reply, "And we are all together, goo goo g'joob?"
William Hartnell originally had a much larger role in the story. He eagerly agreed to take part, but his wife informed Barry Letts and Terrance Dicks that his memory was unreliable and that he was no up to filming. As a result, his role was reduced.
Bob Baker and Dave Martin submitted an idea called "Deathworld". In it, the Time Lords are in conflict with a Federation of Evil led by a personification of Death. To avert all-out war, the Time Lords manage to convince the Federation to allow them to send the three Doctors into the Federation's Underworld domain. There, the Doctors will do battle against various realisations of Death - including zombies, the Four Horsemen of the Apocalypse, the Hindu goddess Kali, and the cyclops Polyphemus from Greek mythology - with the victor in the contests determining whether the Time Lords or the Federation of Evil will prevail.
Dave Jervis, who uncredited as part of Crew 9, an in-house BBC videotape camera crew assigned to the production for the studio sequences, would later join the BBC's Video Effects department and regularly be assigned to provide Video Effects for the show.