Micro-series (originally on disneyxd.disney.com & marvel.com)
Amazon; iTunes; MAU Wiki; TV.com; Wikia; Wikipedia; YouTube

This is a series of twenty minisodes, each about 5 minutes long, that premiered online from September 22 to October 11, 2010, before airing on TV. The first minisode premiered both online and on TV the same day, but while the online releases happened daily after that, episodes 2-20 didn't air on TV until October 11 to 15. All of this led up to the October 20 premiere of the TV series The Avengers: Earth's Mightiest Heroes. The microseries was then compiled into five full-length episodes, each containing four minisodes. (If you want to watch the microseries online these days, this is the format in which you'll find it, at least on legit sources, where you'll have to pay a couple bucks per combined episode. Or you could just buy season one of the TV show on Blu-ray.)

Anyway... the microseries introduced the characters and basic situation of the show. Of course there have been lots of Avengers stories over the years in various comics and TV shows and movies and such, so it's not completely new to me. But every incarnation is a bit different. First we meet the superhero Iron Man, aka Tony Stark, head of Stark Industries. He's fighting an organization called HYDRA, which has been trying to take over the world for decades. (It's led by Baron Strucker, once a lieutenant of the Red Skull.) There's also an organization called SHIELD (Strategic Homeland Intervention, Enforcement and Logistics Division), which fights supervillains, I guess, in a more official way than the world's various superheroes. Its director is Nick Fury. Stark Industries used to make weapons for them, but Tony's not into that anymore. (But it seems HYDRA has somehow acquired Stark's weaponry.)

We also meet Bruce Banner, who is dealing with his gamma-ray induced alter-ego, the Hulk, who is essentially a monster (or at least that's how the world sees him). Banner is worried that SHIELD wants to turn various irradiated people into weapons. (There's also a military group called the Hulk Busters to contend with.) But one agent of SHIELD, Hawkeye, has his doubts about whether the Hulk is truly a monster.

Then there's a scientist named Hank Pym, who can make himself very small (bug-sized) or very large, hence the alter egos Ant-Man and Giant-Man. He can also control ants. He's studying a metal called vibranium. And there's Janet van Dyne, a business associate of Pym's, who can shrink herself to bug-size as well, and fly, and fire energy blasts. She's known as the Wasp (and she's my favorite character on the show).

There's the Norse thunder god Thor, who has to worry about his home realm of Asgard, as well as our world, which Asgardians call Midgard. And Asgard is being attacked by an army led by Thor's own brother, Loki.

And in the 1940s, during WWII, a weakling named Steve Rogers was injected with a serum that greatly increased his strength and speed, transforming him into the super-soldier Captain America. He and his sidekick, Bucky, fought the forces of Red Skull (then head of HYDRA), who was using a bridge between our world and Asgard to capture "mythical" creatures to use for his own purposes.

We also see T'Challa, prince of the African nation Wakanda, whose father T'Chaka was the Black Panther (a position held by the king of Wakanda). The nation was usurped by the Man Ape with the help of Ulysses Klaw (who wanted vibranium, which existed within a meteor that crashed in Wakanda).

Then there's another agent of SHIELD, Black Widow, who is engaged in a deal with HYDRA, but she frames Hawkeye for her own traitorous act.

Also, centuries in future, Kang the Conqueror realized that somehow Captain America was disrupting his timeline. Having conquered Earth in his own time (which is now being erased from existence for some reason), he travels back in time to the present to try to conquer the world now, or at least prevent the destruction of his timeline (and also to save the life of his beloved Ravonna). So... all of this sets the stage for the regular series.