The story begins with Takeshi, a teen ward of the Blue Sky orphanage who recently started an American football team with his friends. On the day before his sixteenth birthday, the caretaker produces a letter that was with Takeshi when he arrived. It tells Takeshi that the author would be returning for him when he turned sixteen, and the news sends him running away in fear.
Takeshi arrives at the football field, pursued by his friends, when a giant green crystal falls from the sky and stabs him in the chest. It emits a blinding light that knocks everyone out, whereupon a UFO appears and beams them aboard. When he wakes up, Takeshi is greeted by Rabi, a royal advisor from the planet Apolon. Rabi reveals to Takeshi that he is the last prince of Apolon, spirited away to Earth to avoid death at the hands of General Dazaan. As a member of the royal family, Takeshi can use the "Key Energy" crystal in his chest to control Dai Apolon, the world's guardian robot.
The series proceeds from there, following Takeshi and his friends as they use the robots to defend Earth against the Dazaan army, retrieve the stolen Energy Heart, and avenge the death of his father. Takeshi controls Dai Apolon, which is formed by combining three separate super robots, while his friends operate various UFOs to provide support and cover fire. As Dai Apolon, Takeshi can use his Key Energy powers to grow and merge with the mecha, "wearing" it like cybernetic armor.
Dubbed versions were broadcast in parts of Europe. Five episodes were combined and dubbed in English to create a Compilation Movie called Shadow World.
Dai Apolon demonstrates the following tropes:
- Adaptation Expansion: The anime is derived from the manga Galactic Warrior Apolon by Tetsu Kariya and Shigeru Tsuchiyama, but adds the Humongous Mecha and American football aspects
- Aliens and Monsters: Many of Dazaan's forces take on demonic forms.
- Color-Coded Characters / Five-Man Band: Takeshi and his friends wear football jerseys with distinct colors.
- Combining Mecha: Not only is Dai Apolon made out of three robots which combine into the titular mecha, it also came with three UFO support craft which can also combine with the individual robots.
- Compilation Movie: Shadow World was created from five disparate episodes.
- Cultural Rebel: Dai Apolon's use of American football as a motif is a form of this trope, given that the sport was barely recognized as an amateur sport in Japan in the mid 1970s; it may have been done to add variety to the show. In-story, Takeshi and his friends frequently play football, while Dai Apolon has a football-helmet-shaped head and uses the "Apolon Ball," a football-shaped double-bladed knife.
- Dub Name Change: Shadow World changed the setting to California and gave the protagonists American names.
- Elemental Shapeshifter: Takeshi's ability to control Dai Apolon was vaguely described as this, where he turned into electricity and combined with its circuitry. However, outside of this singular case, Takeshi exhibited no other electricity-based powers.
- Humongous Mecha: Dai Apolon and its individual component mechs qualify.
- Man in the Machine: Takeshi invokes this trope when he uses his energy powers to grow and merge with the Dai Apolon mech.
- Monster of the Week: A staple of each episode.
- Powered Armor: From a certain point of view, Dai Apolon becomes this for Takeshi.
- Sizeshifter: When Takeshi merges with Dai Apolon, stock footage shows his muscles growing to enormous size to fit inside the mech's form.
- Super Robot Genre
- Theme Naming: The three individual robots that form Dai Apolon are named "Header" (which forms the head), "Trungur" (which forms the trunk/torso), and "Legger" (which forms the legs).
- Transformation Is a Free Action: Averted in one episode, when the Monster of the Week brings miniature flying robotic bugs to block the combination ports of the individual robots. When they try to combine, the bugs explode, interrupting the combination sequence.
- Transformation Sequence: Seen each time Dai Apolon is formed.