When the TARDIS lands on a volcanic island in the middle of the Pacific, the Doctor, Polly, Ben and Jamie soon find a passageway down to the lost city of Atlantis. There, the Doctor meets the famous scientist Professor Zaroff, who has concocted a mad plan to raise Atlantis by draining the ocean waters down into the Earth's core, destroying the planet. It is up to Ben and Jamie to raise a revolution amongst the Atlantean Fish People... before Polly is transformed into one herself.

Veteran writer Geoffrey Orme had gotten his start in film in the mid-Thirties before eventually migrating to television in the Fifties. His small-screen credits included The Avengers and Ivanhoe. Around the start of 1966, he submitted a story idea called “The Evil Eye” to the Doctor Who production office. This was rejected by story editor Gerry Davis on April 4th, but dialogue continued between the two and on August 16th, a new serial entitled “Under The Sea” was commissioned. It was planned that this would be the second story to feature Patrick Troughton's new Doctor, immediately following The Power Of The Daleks.

The director originally assigned to “Under The Sea” was Hugh David. In October, however, David complained that the story would be impossible to make on a standard Doctor Who budget. It was therefore decided that David would instead direct The Highlanders, previously the third Troughton serial, which would take “Under The Sea”'s spot in the schedule. “The Imps”, by Galaxy 4 writer William Emms, was commissioned on October 17th and positioned in the spot formerly occupied by The Highlanders as Serial GG. “Under The Sea” was dropped off the schedule entirely, although Orme would continue to work on it.

The director assigned to “The Imps” was Julia Smith, who had helmed The Smugglers at the end of the previous recording block. Emms worked quickly on his scripts, but fell ill during mid-November and was unable to complete necessary rewrites. With “The Imps” intended to go into production in a month's time, Davis and producer Innes Lloyd realised that there was little likelihood that Emms would have enough time to finish work on the serial. They therefore elected to move “The Imps” back one slot and return Orme's story to the schedule in its place: it was now called “The Fish People”, having also borne the title “Atlanta”.

A key change that now had to be made to Orme's scripts was the inclusion of Jamie McCrimmon, a new companion introduced in The Highlanders whose permanent addition to the regular crew was not confirmed until late November. (Hines' option on “The Fish People” was officially taken up on December 13th.) Davis also contributed to the rewrites. Some details of the storyline were altered or deleted: Zaroff's insanity was originally caused by the death of his wife and children in an automobile accident, while Ara's role was expanded by merging her with another character called Ebon (the excision of Nola was also contemplated for the same reason). Zaroff was at one point assisted by a female scientist called Steen.

It was around this time that Lloyd decided to write Ben and Polly out of the series. He and Davis felt that Polly was not working well, and did not believe that Ben would succeed without her. Additionally, they found that Jamie was proving to be the more interesting of the three companions. Another change which the production team planned to effect was to the opening titles, which had been unchanged since Doctor Who debuted in November 1963. The introduction of the new title sequence was contemplated for “The Fish People”, but was eventually delayed for two stories until The Macra Terror.

By the time location filming began on Serial GG, it had acquired its final title of The Underwater Menace. Establishing shots of the volcanic island were captured on December 12th at Windspit Caverns near Worth Matravers in Dorset, and the series' regulars were present there the next day to record the story's opening and closing sequences. From the 14th to the 16th, cast and crew repaired to the Ealing Television Film Studios for more filming. The centerpiece here was a variety of scenes requiring the facility's water tank, including most material involving the Fish People plus the drowning of Zaroff.

Because of the late completion of Orme's scripts, a planned New Year's Eve start date for the studio recording of The Underwater Menace was pushed back a week, to January 7th. The result was that Doctor Who was now being taped just one week ahead of transmission, an undeniably precarious situation which permitted the production team little room for error. As usual, The Underwater Menace was recorded on consecutive Saturdays in Riverside Studio 1.

The broadcast of this episode on January 14th sparked an angry letter the following Monday from Mrs N Safford of the National Society for the Welfare of Children in Hospitals. Safford was concerned that the sight of Polly undergoing an operation to turn her into a Fish Person would scare children with upcoming surgeries, who might think that they also risked waking up to find themselves transformed into monsters. Lloyd refuted this argument, noting that the fantastical nature of The Underwater Menace made it clear to viewers that the story's events were not realistic.

The normal recording of part four on January 28th was, unusually, prefaced by the taping of its very last scene: out-of-order recording was still a rarity for Doctor Who. This episode also featured the last appearance of the Doctor's stovepipe hat -- the more overtly clownish elements of Troughton's costume had been gradually reined in since his debut in The Power Of The Daleks.

The Underwater Menace was the last Doctor Who serial directed by Smith. She continued to direct for programmes including Z Cars and Dr Finlay's Casebook before stepping up into the producer's chair in the Seventies, subsequently taking the reins of such shows as Angels and EastEnders. Meanwhile, because of the difficulties posed by The Underwater Menace, Davis did not ask Orme for additional story ideas. The writer was now winding down his scriptwriting career anyway, his major concern thereafter being a series of biographical audio tapes. Orme died in 1978.

• Doctor Who: The Handbook: The Second Doctor by David J Howe, Mark Stammers and Stephen James Walker (1997), Virgin Publishing, ISBN 0 426 20516 2.
• Doctor Who: The Sixties by David J Howe, Mark Stammers and Stephen James Walker (1992), Virgin Publishing, ISBN 1 85227 420 4.
• Doctor Who Magazine #209, 16th February 1994, “Archive: The Underwater Menace” by Andrew Pixley, Marvel Comics UK Ltd.
• Doctor Who Magazine Special Edition #4, 4th June 2003, “Good Vibrations” by Andrew Pixley, Panini Publishing Ltd.