The Guardians have smashed Ronan the Accuser's infinity stone,being the power stone, banishing him to earth. He needs the pieces back to make it, which are all across the galaxy. Then, he uses his last powers to break Loki out of prison so he can teleport them to space to find the pieces of the source. In return, Ronan promised to destroy the heroes. Loki breaks the rest of the villains out of prison, and starts the plan. Star-Lord talks to Nick Fury, who rounds up the heroes and sends them to space.
In level 2, you first rebuild Spider-Man and his team's ship and reunite them with the other heroes at Nova Core base, until it turns out the base's security is being wired by Electro and Michael Korvak causing a lock down. Once you've defeated them, the Nova Core give locations of where the pieces of the infinity stone are.
In level 3, Red Skull and Super-Skrull cause a breakout on a prison planet, where the heroes go to defeat them, only to figure out it was a distraction to let villains from levels 3-7 get a head start. In said levels 3-7 the heroes have to find pieces of the stone and defeat old enemies to do so.
After level 7, Iron Man shows back at Nova Core Base that there are only 5 pieces left, until Spider-Man notes they have all the pieces to make the stone by putting it together (just before its power blows him up). Star-Lord tells them that there are 6 different Infinity Stones. He also tells them about Thanos having the gauntlet that holds all of them, which leads some of the heroes to go fight him, only finding out that the Stones were stolen from him, and he'll do anything to get them back, including help the heroes, wich Hawkeye notes is not a bad idea.
Deadpool plays his role from the first game, along with Agent Coulson's.
Certain characters can use Super-Build, a Master Build-like ability where you combine three selected items to make one you need.
The level Hulks Smash! is based off of the Hulk and the Agents of SMASH TV show
Stealth Suit Captain America is made by Minifigs4u. Here is the page were you can check him out.
Ultron is the most powerful character, and is unlocked by purchasing all villains, then going to the helicarier and doing Ant-Man's mission, then using the key he gives you in the mission on Ant-Man's lab, then going to the computer and entering NoStringsOnMe, then doing a computer hack to where Ultron comes in and you have to pay 1,000,000,000,000 studs.
Like LEGO Batman 3: Beyond Gotham has characters singing the 1966 batman theme song in their honor, LMS2 has the characters singing the Spider-Man theme. (Spider-Man, Spider-Man, does whatever a spider can)
3 of the extra missions are called Spider-Verse (1, 2, or 3) which have players going through different worlds of Spider-Man.
Webby reads the comic stories for the Spider-Verse levels.
There is a full list of characters so far, being edited by JJpivotz, here!
The regular DLC pack
The Big Hero 6 DLC (Exclusive with Xbox 360/1 pre-order)