Earth is invaded by an interstellar terrorist group, Big Fire (the Gargoyle Gang in the American version), led by Emperor Guillotine. Guillotine spends most of his time in a multicolored space ship hidden at the bottom of Earth's ocean, from which he issues his orders. The group has been capturing scientists to create an army of monsters to help them conquer Earth. A boy named Daisaku Kusama (Johnny Sokko in the American version) and a young Unicorn peacekeeping agent named Jūrō Minami (Jerry Mano in the American version) are shipwrecked on an island after their ship is attacked by the sea monster Dakolar and subsequently captured by Big Fire. They flee onto an elevator leading to a complex where a Pharaoh-like giant robot is being built by captive scientist Lucius Guardian, who gives Daisaku and Jūrō its control device. Guardian helps them escape before he is shot to death; before he dies, he triggers an atomic bomb which destroys the base. The radiation activates the robot, which now obeys only Daisaku. The boy is invited by Jūrō and his chief, Azuma, to join Unicorn and fight Big Fire with Giant Robo.

• Mitsunobu Kaneko as Daisaku Kusama/Johnny Sokko
• Toshiyuki Tsuchiyama as Giant Robo/Giant Robot
• Koichi Chiba as Narrator
• Akio Ito as Juro Minami/Jerry Mano
• Shozaburo Date as Chief Azuma
• Tomomi Kuwabara as Mari Hanamura
• Hirohiko Sato as Emperor Guillotine
• Yumiko Katayama as Mitsuko Nishino
• Bobbie Byers - Daisaku Kusama/Johnny Sokko
• Ted Rusoff - Juro Minami/Jerry Mano
• Paul Brown
• Mark Harris

In the American version of the series, the Gargoyle Gang is an ambitious, but incompetent, terrorist group with a high mortality rate. They wear a combination of Soviet and Wehrmacht military uniforms, Central American guerrilla clothing and Italian designer sunglasses. The members of the gang all have explosive devices implanted in their bodies that are to be detonated instantly if they are captured.
In each episode, the Gargoyle Gang sends a monster to attack its enemies (the Japanese version's names are listed first, followed by the American version's names):
• Dakolar (Dracolon): Appears in Episodes 1 and 11; swims, has tentacle arms and spits sand.
• Globar (Nucleon and the Radion Globe): Appears in Episodes 2 and 20; resembles a walking limpet mine.
• The Satan Rose (The Gargoyle Vine): Appears in Episodes 3 and 17; powers include rapid growth, constricting tentacle-like vines, suction flowers and lava bombs.
• Lygon (Ligon-Tyrox): Appears in Episodes 4 and 10; powers include a forehead horn drill, mouth flames, a wrecking ball and swimming.
• Gangar (The Gigantic Claw): Appears in Episodes 5 and 18; powers include flight, missiles and a rope.
• Dorogon (Dragon and Staaker): Appears in Episodes 6 and 21; powers include flight, swimming, missiles, invisibility and the abilities to both consume aircraft and ocean vessels and grow and shrink in size (the last with the aid of a special device).
• Ikageras (Scalion): Appears in Episodes 7 and 26; powers include swimming, hurricane winds and acid spray.
• Doublion (Double Head): Appears in Episode 8; powers include head rotation and both a sticky petroleum-based liquid and flames that are emitted from each of its two mouths.
• Sparky (Tentaclon): Appears in Episodes 9 and 22; powers include levitation, electric tentacles and rays.
• Unbalan (Amberon): Appears in Episode 12; powers include self-mummification, resistance to electricity and photosynthesis.
• Gummons (Opticon in the American version, Opticorn in the American version's title): Appears in Episodes 13 and 26; powers include levitation, retractable legs, a vacuum, a searchlight and an energy ray.
• Iron Power (Flying Jawbone): Appears in Episode 14; powers include flight, sharp pointed teeth and body separation.
• Icelar (Igganog): Appears in Episodes 15, 24 and 26; powers include burrowing, freezing winds and low body temperature.
• GR-2 (Torozon): Appears in Episodes 16 and 19; powers include an electric head boomerang, burrowing and eye lasers.
• Calamity (Cleopat): Appears in Episode 22; made with armor that reacts to long-range attacks.

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