William Hartnell (Dr Who), Carole Ann Ford (Susan), Jacqueline Hill (Barbara), William Russell (Ian)
George Coulouris (Arbitan) ; Martin Cort [1,6], Peter Stenson [1,6], Gordon Wales  (Voords); Robin Phillips (Altos) [2-6], Katherina Scholfield (Sabetha) [2-6], Heron Carvic (Voice of Morpho) , Martin Cort (Warrior) , Edmund Warwick (Darrius) , Francis de Wolff (Vasor) ; Michael Allaby, Alan James, Peter Stenson, Anthony Verner (Ice Soldiers) ; Henley Thomas (Tarron) [5-6], Michael Allaby (Larn) [5-6], Raf de la Torre (Senior Judge) , Alan James (First Judge) , Peter Stenson (Second Judge) , Fiona Walker (Kala) [5-6], Martin Cort (Aydan) , Donald Pickering (Eyesen) [5-6], Alan James (Guard) , Stephen Dartnell (Yartek) 
The travellers alnd on an island on the planet Marinus, where the sand is glass and the sea is acid. The TARDIS is captured by Arbitan, Keeper of the Conscience of Marinus, a machine that controls the island absolutely fairly. But four of the five keys that make it function is lost. The Doctor and his companions go to serach for them. On their return with the keys they find Arbitan dead, murdered by Yartek, leader of the Voords, who now control the island. Ian is forced to hand over the four hard won keys, but one is a fake - which causes the machine to explode, blowing itself and the Voord to pieces and freeing the inhabitants of Marinus from its domination.
- All episodes exist as 16mm telerecordings.
- Negative film prints of all episodes were recovered from BBC Enterprises in 1978.
- An Arabic print of "The Sea of Death" is held by the BBC.
- Terry Nation wrote this story as a replacement to The Red Fort, a story that was to be set during the Indian Mutiny.
- William Hartnell does not appear in The Screaming Jungle or The Snows of Terror as the actor was on holiday during the filming of these episodes. This was the first time the lead actor had been allowed to be absent in this way; nonetheless, Hartnell receives screen credit for these episodes. His co-stars will also take time off for holiday during production of upcoming stories.
- This story contains a controversial scene in which it appears that Vasor attempts to rape Barbara.
- Darrius is never referred to by name but his name appears in the show's credits.
- Stephen Dartnell was cast as Yartek, the Voord leader. A few weeks later, he appeared in The Sensorites as the troubled astronaut John.
- Initially it was hoped that the Voords would catch on with young viewers in the same way the Daleks had inspired Dalekmania, with toys, books, and other merchandise. This, however, did not come to pass.
- This is the first story to feature a model TARDIS materialisation.
- Smalls parts of the original film negative for this story were damaged. For the 2009 DVD release, computer imagery was used to restore these small scenes. An example of this is in episode two when Barbara sees Altos in the "real world" (at 0:12:24 on the DVD); according to the text commentary, the first few seconds of the scene had to be recreated via computers to bypass the damaged part of the negative. Episode 4 used an off-air soundtrack recording and a short piece of recycled footage to recreate a short piece of dialogue lost from the master negative; on the DVD, this occurs at 0:14:08.
- Between episodes 2 and 3, the BBC launched BBC Two, a second network. Episode 3, therefore, was the first episode to be aired under the branding BBC One.