The Avengers: Earth's Mightiest Heroes, on Disney XD
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Well, before the series actually started, there was a microseries (20 minisodes roughly 5 to 6 minutes each) that I never got around to watching when they were first released. But I did finally watch them online when I started this review of the series (after having seen the first few regular episodes).

Season One
As you can tell from the microseries, there are a lot of disparate elements that are all ultimately connected. I quite like how much thought has apparently been put into all of this. Anyway, in the two-part series premiere, 74 of the world's most dangerous supervillains escape from SHIELD's four prisons (the Vault, the Cube, the Big House, and the Raft), which had been predicted by one of the prisoners, Mad Thinker, during the microseries. The first villain to make trouble is Graviton, and Nick Fury tries to fight him, but so do several heroes: Iron Man, Thor, Wasp, Ant-Man (aka Giant-Man), and Hulk. After they take down Graviton, Iron Man gets the other heroes to join him in a new team called the Avengers, though they don't exactly get along that well. Hulk leaves pretty soon, because of his own problems, but while they're searching for him, they discover Captain America, frozen in ice since the 1940s, and he joins the team, too. A bit later, the team helps T'Challa (the new Black Panther) defeat Man Ape and reclaim his throne in Wakanda, but he then sets up a council to rule his country, while he joins the Avengers. Hawkeye, a former agent of SHIELD, also eventually joins the Avengers. (He spent the early part of the season trying to catch Black Widow, to prove his innocence, but... I don't want to spoil how that whole situation turns out.)

Throughout the season, there are escaped supervillains to be recaptured. Plus various organizations to fight, like HYDRA and AIM (Advanced Idea Mechanics), and any number of other enemies, including rogue Asgardians the Enchantress and the Executioner. One major story arc involves Kang the Conqueror (from the 41st century) trying to destroy Captain America, for reasons I don't want to go into. And at one point Ultrons, robots created by the Avengers, become a major threat to the world. And um, I guess some of the villains work together, and they're called the Masters of Evil, led by Baron Zemo, one of Captain America's old enemies. But ultimately it turns out they were being used as pawns by the Enchantress, who was working for Thor's brother Loki, who tries to take over all the Nine Realms, of which Earth (aka Midgard) is one. At the end of the season, the Avengers are scattered across the Nine Realms, but they each find new allies, and eventually all reassemble in Thor's home realm of Asgard to defeat Loki. Then, after they return home to Earth, a plot thread for next season is set up, which I won't spoil just yet.

Anyway, the show seems reasonably well written, I like the animation well enough, and I like the characters. (As I said in my review of the microseries, my favorite is Wasp.)

Season Two
So, at the end of last season it was revealed to the audience that Captain America had been replaced by a shapeshifting alien called a Skrull. This season, more of the Avengers are replaced by Skrulls, as part of their plan to invade Earth. Of course, the invasion is ultimately thwarted. Also this season, there is an invasion attempt by aliens called Kree. We also meet an agent of S.W.O.R.D. named Carol Danvers, who acquires super powers and becomes Ms. Marvel, an occasional member of the Avengers. (Well, actually I guess Danvers first appeared in season one, but became an Avenger this season.) Ant-Man quits the Avengers, but eventually returns as Yellowjacket. Another new Avenger is an android called Vision. And Hulk is taken into custody by the military after being blamed for crimes committed by a Red Hulk, but he's eventually released and the true identity of Red Hulk is revealed. Kang also makes more trouble. And the Avengers call upon other heroes to help them on at least a couple of missions, including thwarting the planet-eating alien Galactus. And there's more trouble from the Enchantress, and Loki. And of course lots of other stuff happens throughout the season. (I should mention that I only watched this season on Disney+, and the episodes weren't all in the same order that they originally aired.) Anyway, it was all good stuff, but unfortunately the series was cancelled with some story arcs unresolved.