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!)
Each player sees a 5x5 grid on the card, with nine of the squares coloured green (representing your agents) and one square coloured black (representing an assassin). The assassin is in different places on each side of the card, and three of the nine squares on each side are also green on the other side! Collectively, you need to reveal all fifteen agents — without revealing either assassin or too many innocent bystanders — before time runs out in order to win the game!