According to Ubisoft's blog, the game will be based on the Secret Invasion storyline, the comic which came in 2008. They involve an alien invasion of Earth by shape shifters called the Skrulls.
The Avengers game will feature 20 characters with leads like Thor, Hulk, Captain America, Iron Man and other super heroes and villains from the Marvel universe.
More information is likely to be available at www.AvengersBattleforEarth.com. The link right now redirects to the blog post announcing the game's release.
Ubisoft Quebec is currently also working on the Wii U version of Assassins Creed 3.