The storyline of Army of Two: The 40th Day for the PSP is similar to that of the console version. Players control two commandos on a mission to stop a biological virus from spreading across the city. Along the way, they encounter various enemies and have to make tough moral choices.
The gameplay in Army of Two: The 40th Day is an action-packed top-down shooter that focuses on cooperative play for two players. Either with an artificial intelligence partner or a friend over the PSP's wireless connection, players work together to take down enemies and complete objectives.