The storyline for Army of Two: The 40th Day is similar to its console counterpart. Players take on the roles of a duo of elite operatives, tasked with stopping a sinister plot in an unnamed city. With no clear distinction between good and evil, the player must make tough moral decisions that impact the game world.
The gameplay for Army of Two: The 40th Day is fast-paced and action-packed, with a focus on cooperative play. Players can choose to play solo or team up with an AI partner or a friend over the PSP's wireless connection. With features like weapon upgrades and unlocks, two-player tactics come into play as players work together to overcome challenges.