A team of infiltrators from the triad have penetrated the police force. However, the police have also has sent a team of undercover agents to infiltrate the triad. A decade later, they are still undercover, but their minds have been twisted by the temptation of power, authority and money — and nobody is certain about their original roles anymore. Both groups have learned that they have been infiltrated and want to ferret out the moles. Every mole has a secret code to lock their identity file, and once the code is cracked their identity is fully revealed. The battle of wits begins now. Either uncovering the enemy or taking the chance to change the identity for good can save their lives as long as nobody knows who is who!

Internal Affairs is an intense elimination game between infiltrators from the Triad and the Police Force Infiltrator. Every player has a set of secret ID cards which determine that player's allegiance. Every player also has a secret code, which when cracked will reveal the player's real ID. At the start of the game, each player has five code cards and chooses three of them to be the secret code that locks the identity file. During their turn, players will try to crack other players' codes to find out who is who and get their team to victory. Due to action cards, though, ID cards can be passed from player to player and this may change each player's allegiance. Players must be quick to win, as otherwise they might end up in the losing team when they least expect it.

Internal Affairs is designed for 5-8 players, but alternative rules have also been included to allow for play with 2-4.