The story focuses on Space Security Agent Marc Cory (Edward de Souza) and his attempts to warn Earth of the Daleks' plan to take over the Solar System.

Nation wrote the episode while considering a Dalek-focused spin-off; he used film character James Bond as inspiration for Marc Cory.

"Mission to the Unknown" was produced by the same team as its predecessor, Galaxy 4, in a five-week period that concluded the show's second production block; the two serials were held back to open the third season.

Contemporary and retrospective reviews were generally positive, with praise for the script and direction, though some viewers were confused that the following serial did not immediately continue the narrative.

Usually alongside The Daleks' Master Plan, "Mission to the Unknown" received print and audiobook adaptations, with off-air recordings used to construct the latter.

In 2019, director Andrew Ireland and University of Central Lancashire students recreated the episode in live-action, replicating the 1960s production values.

On the planet Kembel, Marc Cory (Edward de Souza) and Gordon Lowery (Jeremy Young) of UN Deep Space Force Group 1 are attempting to repair their spaceship to reach their rendezvous when they are attacked by the third crew member, Jeff Garvey (Barry Jackson), who was in a violent state of mind upon waking up in the jungle.

Cory is forced to kill Lowery upon discovering he became infected by a Varga plant and records a message, only to be surrounded by the Daleks and exterminated.

[12] The episode was produced by the same team as its predecessor, Galaxy 4; they were the final to be filmed in the show's second production block, but were held back to open its third season in September.

In writing "Mission to the Unknown", he realised such a story would need a hero and wrote Marc Cory as a "space-age Bond", inspired by the recent release of Goldfinger (1964).

[22] Young had previously starred as Kal in the first Doctor Who serial, An Unearthly Child; he was excited to work with Hartnell again but disappointed when he discovered his absence.

[6] Pre-filming for "Mission to the Unknown" took place on 25 June 1965 at Ealing Studios; the scene in which Garvey mutates into a Varga plant was recorded on 35 mm film.

[2][3] 16 mm film recordings were made available for international sale, but the episode was never sold overseas, and BBC Enterprises withdrew it in 1974.

[4] In The Discontinuity Guide (1995), Paul Cornell, Martin Day, and Keith Topping described the episode as "macho, with a sinister atmosphere".

[27] In The Television Companion (1998), David J. Howe and Stephen James Walker wrote that the presence of the lead actors was "hardly missed" due to Nation's script and Martinus's direction, praising the tense atmosphere and set designs.

A library edition of the audiobook was released by AudioGO in 2011,[28] and Demon Music Group published it as a vinyl record in March 2019.

[29] After writing and producing his doctorate thesis about a 1960s-style recreation of the 2006 Doctor Who episode "Tooth and Claw" at Bournemouth University in 2012,[30][31] Andrew Ireland wrote a proposal to recreate "Mission to the Unknown" the following year and brought it to the University of Central Lancashire (UCLan), but did not follow up;[31] he specifically chose the episode as it was a self-contained story without any of the main cast.

[31] By now an academic and Pro-Vice Chancellor of Digital and Creative Industries at UCLan, he obtained permission for the recreation from the BBC and Nation's estate, writing a passionate email to the latter, who were "very supportive".

[43] Erik Amaya of Comicon.com similarly felt the dated techniques "make the 25-minute story feel slower than it actually is" but this was "the whole point of the project", lauding the accuracy of the recreation.