Everything begins with an easy contract Rios and Salem got when they first arrived in Shanghai. Then, all of a sudden, the city gets attacked by a mysterious force called the "40th Day Initiative". This is the story of Rios and Salem getting out of Shanghai as fast as possible while trying to uncover the secret identity of the attackers. While on their way out, they might also have the opportunity to save hostages that the Initiative has tagged to eliminate. But in the end, it's all about getting out alive while the city gets shattered to pieces.

Army Of Two: The 40th Day is a fast-paced third person shooter with a strong emphasis on organic co-op, multiple strategic possibilities and constant destruction.

The game features new co-op moves that can be performed in order to plan and execute new strategies to flank and surprise the enemy. The weapon customization interface is also back, with more weapon parts, paint jobs, and add-ons that will allow multiple tactical possibilities on the battlefield.

Mercenaries don’t have any Rules of Engagement, but they do have values. Several co-operative choices will have to be made throughout the story. It will be up to the players to decide if they want to save innocents on their way out of Shanghai or if they're in only for the money...