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.
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's scenes were filmed at BBC's Ealing Studios and not in a garage or a garden shed as fan myth would have it. However the promotional photographs were taken at Hartnell's home in Kent. First in the back garden, and then in the garage. This would be the only time that all three actors would appear together.
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.
Jamie was supposed to return, and he would have had a romantic subplot with Jo. However, Frazer Hines was busy with Emmerdale Farm (1972) and his lines were given to Sgt. Benton. There were also talks of Wendy Padbury returning as Zoe, but Jon Pertwee vetoed it, as he felt that too many returning characters would be distracting.
There was initially tension between Patrick Troughton and Jon Pertwee, as they had radically different acting styles. Troughton was fond of ad-libbing, while Pertwee believed in sticking to the script. They eventually overcame their differences.
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.
The idea of the Doctor sharing an adventure with his past selves was suggested by fans and others at the BBC. Barry Letts had dismissed this as being too fannish a notion, but in early 1972 he and Terrance Dicks decided that a multi-Doctor story would be an excellent way in which to celebrate Doctor Who (1963)'s decade on the air.
This is the first of three mainstream return appearances by Patrick Troughton as the Second Doctor. His next two were in Doctor Who: The Five Doctors (1983) and "The Two Doctors". By extension, this is the only story featuring William Hartnell and the first for Patrick Troughton to still exist on the original master video tapes they were shot on. (Their runs as the stars of the show only exist as Telerecordings.)
Although Nicholas Courtney had previously acted alongside William Hartnells Doctor in the first 4 Episodes of the 1965/66 story The Daleks Master plan, this is the first time he Plays The Brigadier opposite the 1st Doctor.