Codenames Duet keeps the basic elements of Codenames — give one-word clues to try to get someone to identify your agents among those on the table — but now you're working together as a team to find all of your agents. (Why you don't already know who your agents are is a question that Congressional investigators will get on your back about later!)
To set up play, lay out 25 word cards in a 5×5 grid. Place a key card in the holder so that each player sees one side of the card. Each player sees a 5×5 grid on the card, with nine of the squares colored green (representing your agents) and three squares colored black (representing assassins). Three of the nine squares on each side are also green on the other side, one assassin is black on both sides, one is green on the other side and the other is an innocent bystander on the other side.
Collectively, you need to reveal all fifteen agents — without revealing an assassin — before time runs out in order to win the game.