On 21 June 2004, the BBC announced in a press release that a new series of Hitchhiker's based on the third novel would be broadcast as part of its autumn schedule, produced by Above the Title Productions Ltd. The episodes were recorded in late 2003, but actual transmission was delayed while an agreement was reached with The Walt Disney Company over Internet re-broadcasts, as Disney had begun pre-production on the film. This was followed by news that further series would be produced based on the fourth and fifth novels. These were broadcast in September and October 2004 and May and June 2005. CD releases accompanied the transmission of the final episode in each series.
The adaptation of the third novel followed the book very closely, which caused major structural issues in meshing with the preceding radio series in comparison to the second novel. Because many events from the radio series were omitted from the second novel, and those that did occur happened in a different order, the two series split in completely different directions. The last two adaptations vary somewhat—some events in Mostly Harmless are now foreshadowed in the adaptation of So Long and Thanks For All The Fish, while both include some additional material that builds on incidents in the third series to tie all five (and their divergent plotlines) together, most especially including the character Zaphod more prominently in the final chapters and addressing his altered reality to include the events of the Secondary Phase. While Mostly Harmless originally contained a rather bleak ending, Dirk Maggs created a different ending for the transmitted radio version, ending it on a much more upbeat note, reuniting the cast one last time.
The core cast for the third through fifth radio series remained the same, except for the replacement of Peter Jones by William Franklyn as the Book, and Richard Vernon by Richard Griffiths as Slartibartfast, since both had died. (Homage to Jones' iconic portrayal of the Book was paid twice: the gradual shift of voices to a "new" version in episode 13, launching the new productions, and a blend of Jones and Franklyn's voices at the end of the final episode, the first part of Maggs' alternative ending.) Sandra Dickinson, who played Trillian in the TV series, here played Tricia McMillan, an English-born, American-accented alternate-universe version of Trillian, while David Dixon, the television series' Ford Prefect, made a cameo appearance as the "Ecological Man". Jane Horrocks appeared in the new semi-regular role of Fenchurch, Arthur's girlfriend, and Samantha Béart joined in the final series as Arthur and Trillian's daughter, Random Dent. Also reprising their roles from the original radio series were Jonathan Pryce as Zarniwoop (here blended with a character from the final novel to become Zarniwoop Vann Harl), Rula Lenska as Lintilla and her clones (and also as the Voice of the Bird), and Roy Hudd as Milliways compere Max Quordlepleen, as well as the original radio series' announcer, John Marsh.
The series also featured guest appearances by such noted personalities as Joanna Lumley as the Sydney Opera House Woman, Jackie Mason as the East River Creature, Miriam Margolyes as the Smelly Photocopier Woman, BBC Radio cricket legends Henry Blofeld and Fred Trueman as themselves, June Whitfield as the Raffle Woman, Leslie Phillips as Hactar, Saeed Jaffrey as the Man on the Pole, |Sir Patrick Moore as himself, and Christian Slater as Wonko the Sane. Finally, Adams himself played the role of Agrajag, a performance adapted from his book-on-tape reading of the third novel, and edited into the series he created some time after the author's death.
Tertiary, Quandary and Quintessential Phase Main cast:
- Simon Jones as Arthur Dent
- Geoffrey McGivern as Ford Prefect
- Susan Sheridan as Trillian
- Mark Wing-Davey as Zaphod Beeblebrox
- Stephen Moore as Marvin, the Paranoid Android
- Richard Griffiths as Slartibartfast
- Sandra Dickinson as Tricia McMillan
- Jane Horrocks as Fenchurch
- Rula Lenska as the Voice of the Bird
- Samantha Béart as Random
- William Franklyn as The Book