Main: Slightly naive Tau fire warrior. Has a tendency to bumble into things, and the few episodes he doesn't get the heroes into trouble that way his bumbling is what ultimately saves the day by sheer dumb luck.
Supporting cast: A harsh and somewhat boisterous Space Marine who frequently is used as an aesop about being nice to people, though he himself never learns permanently (classic aesop amnesia). A kroot deadpan snarker who is dangerously genre-savvy though rarely acts on it, as he prefers to sit around and eat pretty much anything he happens across (played for laughs, may get used as an aesop that he, of course, forgets after the end of the episode). A tsundere gue'vesa auxiliary as the token girl and romantic interest who frequently takes advantage of the fact that the lead sucks in close combat to bludgeon him upside the head. An Eldar death jester as the resident badass among morons, there purely for popularity's sake and is more of a "wild card" than an actual hero, frequently off doing his own thing rather than helping out.
Villains: A bumbling Chaos Lord and his Chaos Sorcerer lieutenant. They never, ever win. Ever. And frequently go flying off into the warp shouting "I'll get you next time, fire warrrrrrriiiiiooooooorrrrr!" The Chaos Lord is the dumb brute who, while strong, mucks things up constantly. The Chaos Sorcerer is sly, but cowardly. He is constantly trying to off the Chaos Lord and take his place but it never works and frequently backfires in a hilarious fashion (he does this at least once every single episode they appear in to no avail). His constant treachery sometimes undermines his other schemes, as he has trouble keeping them all straight. The two are later upstaged by their boss, a daemon prince, who is introduced in a standard "newer, bigger, nastier bad guy" season finale.
Cerebus Syndrome strikes at this point.
When the daemon prince gets killed by HIS boss (a Lord of Change) for failing. The Lord of Change whips the Lord and the Sorcerer into line and has them start doing actually evil things, which they find revolting, and they start plotting to try and get out from under the daemon's thumb.
The sudden competency of the villains take the heroes by complete surprise and they almost all die. The kroot makes a heroic sacrifice (he's genre savvy, the heroes best friend, AND the dead pan snarker so he's the one who dies) to save them. The tau starts loosing is naivete, the gue'vesa gets raped, the space marine gets ordered to betray them all (which he does with heavy hearts) and is killed by the tau, and the death jester starts acting more like an ass than a real friend.
In keeping with the sudden and inexplicable adherence to the GRIMDARK, the Eldar should turn out to be a dick who was playing them for saps all along. He slips into quasi-villain territory, requiring someone to replace him considering you can't leave the romantic leads alone on the team or then the relationship might ACTUALLY GO SOMEWHERE. Is this real or are you just jacking of?
Protagonist: Young Imperial Guard soldier, who is a conduit for the emperor himself (think Yu-Gi and the Pharoh). Campaign begins on an IG training world which becomes under attack by Chaos. Everyone thinks they were just up to no good, but it turns out they were sent to kill Protagonist before he learns to control his Emprah-power. Protagonist isn't an amazing soldier, but he has the gift of Boundless Energy and Enthusiasm, which drives all main shonen characters.
The Eldar intervene to stop the assassination (knowing that such an event would end the universe blah blah). Protagonist saves the life of an Eldar ranger, who now owes him a life-debt. Ranger is aloof and arrogant, which fills the requirement of the aloof and arrogant character. Protagonist quickly befriends Ranger, disinterested in their racial differences.
When the rest of the IG see Protagonist with the Xenos, they ostracise and try to kill him. The two flee the camp (using super Ranger sneakin' powers).
Protagonist and Ranger try to find a way off the planet. They encounter some Space Marines(who are marines malevolent level bad) mopping up a small ork incursion. The only ork left is a mekboy, who has been strung up as the soldiers torture him. Protagonist is shocked and disgusted, and runs at them screaming "LEAVE HIM ALONE YOU BASTARDS!". Psychic energy spews out of him and, for a second, he is the emperor incarnate. He decimates the marines and frees the ork.
Mekboy is enthralled by the power of Protagonist, and decides to offer them safe passage off the planet because "dat humie is da strongest". Leading them to the small transport he has, Mekboy is surprised that it doesn't work when they all get in. Of course, ork technology is largely fueled by psychic energy, in the form of "er, this'll work won't it?". The Mekboy tells the other two that they have to believe that the ship will get them off the planet.
In the final momentous scene of the first story arc, the three characters close eyes and hold hands, willing the ship to move. Slowly it inches off the ground, and flies into space. And so begins their epic journey...
The trio crash land on an agri-world. Using eldar cloaking technology, Ranger and Ork are disguised as guardsmen. They learn from the local farmers that they are being plagued by Dark Eldar raiders. While searching for the secret base camp of the raiders they stumble upon a long forgotten dreadnought in stasis. When they awaken him he quickly sees through the eldar cloaks, but stops when he looks upon the Protagonist and exclaims "It is the Emperor himself!". The dreadnought begins to tell them of the ancient prophecy of the emperor reborn, but is cut short when the dark eldar appear and blow him up. It turns out the leader of the Dark Eldar is Rangers brother. The heroes are subdued and taken to the main camp to be tortured and killed. Ranger challenges Brother to single combat. Brother accepts. Ranger manages to defeat Brother using the ancient lessons passed down to them that Brother was too arrogant to acknowledge.
Ranger is poised to make the killing blow, but stops, saying he cannot kill someone that he loves.
Brother realizes the error of his ways, and that he has hardened his heart to kinship and togetherness. In a valiant act, he frees the heroes and the four of them fight their way out of the camp, escaping on a Dark Eldar raider.