- Anton Phillips - Dr. Mathias
- Catherine Schell - Maya
- Catherine Schell - Servant of the Guardian
- Joan Collins - Kara
- Peter Duncan - Cantar
- Judy Geeson - Regina Kesslan
- Peter Bowles - Balhor
- Bernard Kay - Humanoid
- Leigh Lawson - Zarl
- Bernard Cribbins - Captain Michael
- Oliver Cotton - Spearman
- Joanna Dunham - Vana
- Guy Rolfe - Magus
- Billie Whitelaw - Zamara
- Marc Zuber - Security Lieutenant
- Peter Cushing - Raan
- Stacy Dorning - Zova
- Gareth Hunt - Eagle Pilot (uncredited)
- Nick Brimble - Ray Torens
- Freddie Jones - Charles Logan
- Michael Culver - Pete Irving
- Philip Madoc - Commander Gorski
- Geoffrey Bayldon - Number Eight
- Valerie Van Ost - Main Mission Operative
- Don Fellows - GTV Newscaster
- Richard Johnson - Lee Russell
- Christopher Lee - Captain Zandor
- Caroline Mortimer - Dione
- Maxine Audley - Theia
- Kevin Stoney - Talos
- Carolyn Courage - Prima ragazza
- Maggie Wright - Main Mission Operative (uncredited)
- Mary Miller - Freda
- Eric Carte - Collins
- John Hamill - Dominix
- David Ellison - Ted
- Giancarlo Prete - Dan Mateo
- Isla Blair - Female Alien
- Isla Blair - Carla
- Alex Scott - Aarchon
- Lon Satton - Benjamin Ouma
- Eva Reuber-Staier - Jane
- Carla Romanelli - Melita Kelly
- Lawrence Trimble - Steve Abrams
- Lea Brodie - Hilary Preston
- Leo McKern - Companion
- Rula Lenska - Joan Conway
- Orso Maria Guerrini - Luke Ferro
- Barbara Kellerman - Monique Bouchere
- Michael Sheard - Darwin King
- Dennis Burgess - Neman
- Susan Jameson - Juliet Mackie
- Laurie Davis - Stewardess
- Laurie Davis - Main Mission Operative (uncredited)
- Gwen Taylor - Nurse
- Aubrey Morris - Petros High Priest
- Paul Antrim - Bill Lowry
- Robert Russell - Hadin
- Gerald Staddon - Male Mute
- Louise Jameson - Survivor
- Ron Tarr - Survivor
- Gianni Garko - Tony Cellini
- Douglas Wilmer - Commissioner Dixon
- Julian Glover - Jarak
- Cyd Hayman - Sue Crawford
- Jeremy Kemp - Ernst Linden
- John Shrapnel - Jack Tanner
- Anthony Valentine - Male Alien
- Margaret Leighton - Arra
- Ian McShane - Anton Zoref
- Gay Hamilton - Eva Zoref
- Roy Scammell - Jim Nordstrom
- Brian Blessed - Cabot Rowland
- Brian Blessed - Mentor
- Paul Jones - Mike Ryan
- Jon Laurimore - Smitty
- Jan Harvey - Alpha News Service Girl
- James Smillie - Baxter
- Alan Meacham - Sandos
- James Fagan - Ken Johnson
- Gary Waldhorn - Winters
- Max Faulkner - Ted Clifford
- Emily Bolton - June
- Derek Anders - Wayland
- Hilary Dwyer - Laura Adams
- Anthony Nicholls - James Warren
- Anouska Hempel - Annette Fraser
- Lisa Harrow - Anna Davis
- Gerard Paquis - Lew Picard
- Margaret Inglis - Mirella
- Peggy Ledger - Old Lady
- Roger Bizley - MacDonald
- Shane Rimmer - Eagle Pilot (voice)
- Shane Rimmer - Bannion (voice)
- Shane Rimmer - James Kelly
- Shane Rimmer - Maintenance Section Voice (voice) (uncredited)
- Norwich Duff - Texas
- Michael Sharvell-Martin - Brian the Robot
- Annie Lambert - Operative
- Barbara Wise - Girl
- John Standing - Pasc
- Michael Gallagher - Etrec
- Veronica Lang - Lyra / Maurna
- John Alkin - Johnson
- Anthony Forrest - Carson
- Terry Walsh - Rescue Eagle Pilot
- Godfrey James - Alien Transporter
- David Jackson - Alien Strong
- Patrick Mower - Dave Reilly
- Willoughby Goddard - Taybor
- Laraine Humphrys - Karen
- Rita Webb - Slatternly Woman
- Jack Klaff - Security Guard
- Stuart Damon - Guido Verdeschi
- Stuart Damon - Parks
- James Leith - Guard
- Ina Skriver - A
- Sarah Douglas - B
- Robert Rietti - Sphere (voice)
- James Laurenson - Patrick Osgood
- Pamela Stephenson - Michelle Osgood
- Lloyd McGuire - First Engineer
- Brendan Price - Security Guard
- Nova Llewellyn - Alphan Woman
- Albin Pahernik - Lizard Animal
- Albin Pahernik - Creature
- Albin Pahernik - Maya / Creature
- Albin Pahernik - Space / Kreno Animal
- Peter Porteous - Petrov
- Andrew Lodge - Grasshopper
- Trevor Thomas - Eagle Pilot
- Tony Osoba - Security Guard
- Lynne Frederick - Shermeen Williams
- Stuart Wilson - Vindrus
- Nicholas Campbell - Eddie Collins
- Brian Osborne - Mr. Potter
- David Prowse - Cloud Creature
- Deborah Fallender - Carolyn Powell
- Jess Conrad - Mark Sanders
- Roy Marsden - Alien Invisible
- Roy Marsden - Crael
- Anthony Stamboulieh - George Crato
- Laurence Harrington - Jackson
- Laurence Harrington - Stewart
- Gregory de Polnay - Peter Garforth
- Lydia Lisle - Sally Martin
- Lucinda Curtis - Tessa
- Dallas Adams - Sam
- Sarah Bullen - Kate
- Nicholas Young - Peter Rockwell
- Drewe Henley - Joe Ehrlich
- Billy J. Mitchell - Professor Hunter
- Cher Cameron - Louisa
- Al Lampert - Ken Burdett
- Robert Sheedy - Henry
- Patrick Westwood - Dr. Shaw
- Ken Hutchison - Greg Sanderson
- Jeffery Kissoon - Ben Vincent
- Jeremy Young - Jack Bartlett
- Toby Robins - Diana Morris
- Carolyn Seymour - Eva
- Nigel Pegram - Cernik
- James Snell - Stevens
- Gyearbuor Asante - Guard
- Lee Montague - Dorzak
- Jill Townsend - Sahala
- Kathryn Leigh Scott - Yesta
- Yasuko Nagazumi - Yasko
- Paul Jerricho - Security Guard
- John Judd - Security Guard
- Hildegard Neil - Elizia
- Cassandra Harris - Controller / Sares
- Angus MacInnes - Jelto
- Michael Dickinson - Blake Maine
- Arthur White - Kinano
- Nadim Sawalha - Zoran
- John Hug - Bill Fraser
- John Hug - Fraser
- Karl Held - Jerry Travis
- Sam Dastor - Ed Spencer
- Walter McMonagle - Les Johnson
- Roy Boyd - Joe Lustig
- Patrick Troughton - Archon
- Ann Firbank - Consul Varda
- Gerry Sundquist - Malic
- Alibe Parsons - Alibe
- Michael Halsey - Dorcon Soldier
- Roy Dotrice - Gerald Simmons
- Hazel McBride - Medical Officer
- Hal Galili - Voice
- Harry Fielder - Medic (uncredited)
- Harry Fielder - Security Guard (uncredited)
- Harry Fielder - Gerry (uncredited)
- Harry Fielder - Security Guard George (uncredited)
- Harry Fielder - Command Center Technician (uncredited)
- Harry Fielder - Survey Team Member (uncredited)
- Sylvia Anderson
- Fred Freiberger
- Gerry Anderson
- Alan Willis
- Barry Gray
Peter Cushing interpreta a Raan en la serie.
Christopher Lee hace el papel de Captain Zandor.
Don Fellows interpreta a GTV Newscaster en la serie.
Roy Dotrice hace el papel de Gerald Simmons.
Nadim Sawalha interpreta a Zoran en la serie.
John Shrapnel interpreta a Jack Tanner en la serie.
Julian Glover interpreta a Jarak en la serie.
Aubrey Morris interpreta a Petros High Priest en la serie.
Isla Blair interpreta a Female Alien en la serie.
Martin Landau interpreta a John Koenig en la serie.
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