|You know it makes sense
From 1964 to 1967 Gerry and Sylvia Anderson produced the three classic TV series Stingray, Thunderbirds, and Captain Scarlet, using the "Supermarionation" techniques of puppetry. Although created as children's television, their appeal has been far wider. The Andersons had been developing their techniques with earlier series, up to Fireball XL5 (1962) but it was with Stingray that they really hit their stride. Joe 90, which followed Captain Scarlet, has not established quite such a long-term cult.
The three series are interestingly different, and although pretty well everyone who likes one will watch all of them, their appeal varies. One thing they do have in common is a complete rejection of the idea of child characters to identify with. One or two children do appear, but only when they are relevant to the plot. There is no Wesley Crusher or even Jake Sisko.
Stingray is about the adventures of the crew of the eponymous submarine, but also the base staff of the WASPs (World Aquanaut Security Patrol). Although Troy Tempest, the captain, is the only character to get star billing, he is only one character, and not the most interesting, in a memorable group. There is a love triangle. Troy is besotted with Marina, the undersea woman they rescued from slavery, but Atlanta is in love with Troy. Marina cannot speak, and although intelligent apparently isn't able to communicate by writing or sign language either, beyond the "one tap for yes" level, so we don't really know what she thinks. But you get the impression she likes Troy but that's as far as it goes. She is extremely loyal to Troy, but that's not quite the same. If anything she seems to favour Phones, but she's too independent to get tied down, especially to someone who can't even breathe underwater. In later episodes it sometimes looks like Troy is coming round to Atlanta. He doesn't deserve her. In the finale, "Aquanaut of the Year" (where Troy is being honoured with a This Is Your Life programme) Atlanta is asked whether there is anything between them, and makes an unconvincing explanation that they are just friends and Troy isn't the marrying type; and then Troy says he would like to say that—and at this point he is cut short by an emergency. Their love lives are also complicated by jealousies involving outsiders. There's quite a lot of humour.
The stories are often about conflicts with undersea peoples, but these are often caused by misunderstandings, and the WASPs, though ready to use force, tend to seek peaceful resolutions where possible. Even if it isn't, it's rare for anyone you have actually known as an individual character to get killed. Enemies often end up getting arrested. The anonymous Aquaphibians of the tyrant Titan get blown up, though. Not all the undersea peoples are hostile, and there's quite a variety of stories.
Stingray is notable for the adult world portrayed. The characters drink and smoke. In a crisis Commander Shore gets a five-o'clock shadow and is surrounded by dirty coffee cups. The characters have parties. In fact, they have a complete life outside work, and this is the basis of a lot of the characterization. Marineville has a supermarket. The WASPs have an entertainment section, with a jazz band piloting their own submarine. In the finale, Troy has a hangover from a fairly wild party the night before.
On one occasion Commander Shore is getting nightmares about a traumatic episode in the past. He doesn't want to talk about it but Atlanta calls Troy, who gets up in the middle of the night to come over and encourage Commander Shore to talk about it. Who says a strong man can't talk about his feelings? (Of course, this rapidly leads to a new adventure.)
Thunderbirds is generally acknowledged to be the Andersons' masterpiece. (I won't say so much about Thunderbirds as the others, since it's by far the best known.) It is set apart in a number of ways. While both Stingray and Captain Scarlet are about military organizations, Thunderbirds is all about the value of life, and in the first episode Jeff Tracy notes that their equipment is too advanced to be trusted to those who would use it to destroy life. (The military, when they do appear, have their good points but are open to criticism: they almost shoot down Thunderbird 2 in what Jeff calls a "senseless attack".)
Characterization of the Tracys is more limited than in Stingray. The Tracys do have a life outside work, but they're rather like pilots waiting to scramble. (Actually, they are pilots waiting to scramble.) Virgil does his paintings, but he has to be ready to go, and the secrecy of the base limits outside contact. They drink and smoke a bit.
Lady Penelope, however, is much less limited, and in terms of characters she is arguably the star. International Rescue relies on her, and she is more or less Jeff's equal. But they are different. Lady Penelope is far more ruthless than the Tracys, with a soft feline charm that conceals very sharp claws. Her butler/chauffeur Parker, he of the strangulated vowels, is an ex-convict. He's gone straight now—more or less—but is not above using his underworld knowledge for blackmail in a good cause. I've seen it suggested that the Tracys are technocratic professionals whereas she is an amateur, but it's Lady Penelope who has the professional attitude.
There is less love interest in Thunderbirds. Although it isn't made explicit, Scott seems to have a crush on Lady Penelope. She is so out of his league that it could be embarrassing. Alan is apparently dating Tin-Tin, though it's hard to know how serious it is. Tin-Tin is actually a bit of a tech wizard but we seldom see this because she's overshadowed by Brains. In one episode she gets to be Lady Penelope's side-kick, which would have been the way to go if the series had continued.
Captain Scarlet  has a very different appeal. Many people particularly like it, as Gerry Anderson's "darkest" work, and you also see the view that it is more sophisticated in some way. My view is that the concept is fascinating but the actual execution fails to live up to it.
What is the concept? Earth has got itself into a war with the Mysterons, who are based on Mars. This is entirely Earth's own fault. (Earth's unprovoked attack, to which the Mysterons repeatedly refer, was an accident, but not really an excusable one.) The Mysterons seem to exist partly outside our dimension, can control time and space in some ways, and are intent on revenge. Not only are they themselves incomprehensible, but they aren't even fighting the war in a way that makes sense to the Earth people; they issue specific threats, and if they fail, they leave it at that and move on to the next threat. It has been suggested this is rather like terrorism. They replace people and objects with (almost) indestructible replicas which do their will. Creepy.
Captain Scarlet was replaced in this way but due to an accident is restored to his old self, while keeping his Mysteron indestructibility. Logically this is not actually Captain Scarlet, who died, but a replica with a copy of his mind, but this doesn't seem to bother anyone. Captain Scarlet can thus do things involving getting killed, and come back for more.
The war is carried on by Spectrum. Here it gets even weirder. Spectrum is based in Cloudbase, a flying aircraft carrier. It's headed by Colonel White, and has a small air force of Angels, a multi-racial but all-female group. The Mysterons' chief agent is Captain Black, a former Spectrum agent they took over. Captain Scarlet dies and comes back from the dead. Does all this remind you of anything? Gerry Anderson maintained that all of this apparent religious symbolism was accidental, and gave an account of how the various elements were invented. But it's hard to believe there wasn't at least something subconscious, perhaps Jungian archetypes. (Cy Grant, the actor who voiced Lieutenant Green, has suggested an interpretation of Captain Scarlet in allegorical terms.) Oh, and at one point Captain Scarlet saves the day at the last moment by the code letters AMEN. The symbolism is however complicated by the fact that it's Spectrum which committed the Original Sin of attacking the then friendly Mysterons.
Spectrum wins most of the rounds, but not all. Given the Mysterons' power, you get the sense they are just toying with Earth, and that Earth may be losing. Spectrum makes an attempt to talk peace but without success.
The series was unusual in its time for what we would now call its diversity. Spectrum is multi-racial, though we mainly see the white Angels, and Lieutenant Green (a Black man) is, as he himself complains, usually stuck at headquarters. Female fighter pilots were a pretty startling idea in 1967. (Incidentally, in Stingray Commander Shore is a paraplegic; and although he normally stays in Marineville he does once turn up at the controls of Stingray.)
So, Captain Scarlet has a really interesting science-fiction premise. However, the actual stories seem to me not to live up to the possibility. Partly because of long action sequences, they are curiously slow-moving: much more happens in an episode of Stingray. There seems little time for the characters, and they are barely individuals compared to the characters in the other series. Captain Blue is apparently in love with one of the Angels. I can't remember which one and it doesn't make much difference.
Perhaps because of this, the best parts of Captain Scarlet are the ritual sequences. "This is the voice of the Mysterons. We know you can hear us, Earthmen." The killing of the Mysterons' target, then the circles of light, and the camera shifting to show the replica. You never see the replica appear, which makes it all the creepier. It's just there. Some of the episodes had a final song, about Captain Scarlet's indestructibility. "They crash him, and his body may burn/They smash him, but they know he'll return." OK, bad news for the Mysterons, but not great news for Captain Scarlet either. Coming back to Captain Scarlet as an adult, this is something that strikes you. Captain Scarlet is said to be "indestructible" but his suffering and death is real—it just isn't permanent. A series of pictures at the final credits show Captain Scarlet about to be killed in a variety of unpleasant ways. One has to hope he has a good therapist.
Another interesting comparison of Stingray and Captain Scarlet is their use of dream sequences. Captain Scarlet has an episode of the "it was all a dream" type, which tends to leave audiences feel a bit cheated. Stingray, however, uses dreams more imaginatively. In one, the dream is surealistic, with Stingray finding itself miniaturized inside a fishtank. In another (strictly speaking a hallucination rather than a dream), Troy becomes the richest man in the world and his friends sit around addressing him as "Great One". As he himself admits afterwards, he's a bit of a glass-bowl.
While on the subject of openings and closings: although Thunderbirds has a superb opening, with the countdown and scenes from the episode, it faces serious competition from Stingray. Gerry Anderson once said that the extraordinary opening was designed to grab the attention and prevent a channel change. The programme also has the best of all the Supermarionation closing sequences. The fact that it is about the love triangle rather than Stingray's adventures indicates how important the characters and their lives are in this series. Troy sings about his love for Marina: he doesn't even know if it's unrequited, because she can't answer. The scenes are of Troy and Marina. In a couple they are together; Troy is looking at her, but she isn't looking at him. You only notice this sort of thing coming back to it as an adult. There is just one different shot, in which Atlanta looks down lovingly at a mounted photo of Troy; it is remarkably poignant.
Of the three, Thunderbirds stands out as the masterpiece. But I find Stingray far more interesting to watch than Captain Scarlet.
 Joe 90 has some subtle differences from the others. It makes most sense as a sort of wish-fulfilment fantasy: if taken literally the premise that his stepfather lets Joe put his life in danger is rather disturbing. There is a striking and puzzling absence of female characters compared to Stingray, Thunderbirds, and Captain Scarlet (and even Fireball XL5). It was apparently intended to be more character-oriented than previous series, but although it is indeed more character-oriented than Captain Scarlet, it is less so than Stingray (certainly) and Thunderbirds (probably). Also, Joe 90 abandons the purely adult focus which was such a feature of the previous three series. [Return]
 I've seen it stated online that Marina is in love with Troy. I don't know what if any backstory the writers assumed, but I think it's hard to argue that's what is actually portrayed. [Return]
 I'm only going to discuss the original 60s version. The full title is Captain Scarlet and the Mysterons but everyone calls it Captain Scarlet. [Return]
 This sequence was later used in a compilation film as being actual events. This film, which provides a sort of resolution, is quite good but unfortunately seems to be unavailable. [Return]