The story tells the tale of three heroes Hercules, Mercury and Astrea who were taken out of their timestreams to a faraway world, given superpowers and sent back to Earth to protect mankind. Hercules and Mercury were based on the figures from Greek & Roman mythology but Astrea was created for the series. (In-universe, however, the mythical Hercules and Mercury are explicitly based on them)
Space Sentinels provides examples of:
- Aliens Speaking English: Sentinel-1 and Mo.
- All Asians Know Martial Arts: Mercury knows karate (explicitly called such, even though evidence points to him being Chinese), and often uses it to jump-kick robots in the head.
- All Myths Are True: But they aren't all accurate. Most "gods" are actually space aliens (like Anubis), extra-dimensional entities (like Loki), or superpowered humans (like Mercury and Hercules).
- All Your Powers Combined: Morpheus in "The Sinister Sentinel" had all three sentinel's powers - Hercules' strength, Mercury's speed and Astrea's animorphing.
- Animorphism: Of the Voluntary Shapeshifting variety. Astrea is "able to take any living form".
- Distaff Counterpart: As much as giant holographic heads can have a gender at all, Sentinel-7 of the planet Drakon is this to Sentinel-1. She oversees a gender-flipped team of Sentinels, Thon, Valya, and Vaydra.
- Friend to All Living Things: Also Astrea. She can understand all animal languages and can befriend any animal, even ones normally terrified of humans.
- HeelFace Turn: Anubis is willing to forego his vendetta with humans when the Sentinels offer to send him back in time to prevent his people's destruction.
- Latex Space Suit: Mercury and Astrea's costumes.
- Meaningful Name: In "The Time Traveler", the titular character is named Kronos. Chronos was the personification of time in Greek Classical Mythology.
- Mind over Matter: The power of the title villain in the episode "Loki".
- Robot Buddy: Sentinel-1's Maintenance Operator, "Mo" for short.
- Shapeshifter Baggage: There's apparently no size limits to what Astrea can turn into, from mouse to elephant.
- Smurfette Principle: Astrea is the only female character in the main cast. Almost averted when they team up with the Sentinel team from the planet Drakon, which consists of one man and two women, but the women spend the entire episode in stasis, so Astrea is still the only woman in the active team.
- Speed, Smarts and Strength: The Power Trio is composed of Hercules, a Nordic blond male with tremendous strength; Mercury, an Asian male with phenomenal speed; and Astrea, an African female with pleomorphic powers. Since Astrea, in most of her forms, is still the most vulnerable of the three, she's also an excellent tactician.
- Stock Footage: Filmation was very well known for doing this in most of its animated series, and this is no exception. Even conversational shots are reused (which is why the characters' expressions rarely change from a vague smile, even in the most dire of situations). On top of that, they then reused Hercules' design and animation in a completely different show, The Freedom Force (no not that one), part of Tarzan and the Super 7.
- Super Speed: Mercury, who can "match the speed of light".
- Super Strength: Hercules, with the "strength of a hundred men".
- Time-Travel Tense Trouble: In "The Time Traveler":Kronos: How do you know my name?
Astrea: We've met before - or should I say since.
- Token Minority / Token Trio / Twofer Token Minority: Mercury is Asian (implied to be Chinese in "The Wizard of Od" and "The Sorceress"), Astrea is black (her original outfit appears to be Egyptian), and even Hercules is Ambiguously Brown (though he might just have a good tan).
- 20 Minutes into the Future:
- The series takes place in 1985. By this time, a moonbase is under development.
- In "The Time Traveler", the title character Kronos comes from 1999. In that year, an alien race arrived on the moonbase and agreed to share its advanced technology with the peace loving countries of the world. However, Kronos' nation was excluded as it was considered too warlike.
- Walking Shirtless Scene: Hercules.
- Wing Ding Eyes: Mo expresses non-verbal communication with these, especially question marks when confused.
- Your Magic's No Good Here: In "The Wizard of Od", the team visits an alternate dimension called Fansia, where their powers work oddly, when they work at all: Mercury finds himself going super-slowly, Hercules has no super-strength, and Astra can only turn into mythical animals.