Marvel Super Hero Squad: The Infinity Gauntlet

Story related and can't be missed. To access Doom on the Loose! just continue your game and access the free play computer on the right side of the main lobby and it will be at the bottom of the list. It will take all of 20 minutes max to get through as there are very few enemies to fight and 3 sets of heroes to defeat. All in all it should pose no problem if you have any experience with the game.