The world has finally embraced global peace. A network of intercontinental expressways has been established, bridging the continents. Radioactive materials and weapons of mass destruction have been eliminated, removing all threats to peace. Then in a major turn of events, the world stood in silence as a new fear was unleashed and the 'killer 7' was commissioned to handle this new threat to mankind.

In killer 7, players take on the role of Harman Smith an unlikely assassin who leads a life as the "master" who rules over seven alter egos. Switching between his seven different personalities, Harman employs each personality's unique abilities to complete their mission. Now, a powerful underworld kingpin has unleashed violent creatures onto the world called 'Heaven's Smile.' These crazed soldiers, affixed with horrific smiles, have started to wreak havoc on city streets. It is up to the player to interchange Harman's seven 'partners' and utilize the special powers that dwell within them in order to assassinate Kun Lan, the notorious criminal overlord behind all the madness.