Codenames duet keeps the basic elements of codenames - players give one word clues to try to get someone to identify the agents among them. In this version, you're working together as a team to find all of your agents.
You and your partner are on a secret mission to a crowded city. Your objective: to contact 15 agents while avoiding a band of enemy assassins. You know the agents that your partner can contact safely. Your partner knows the agents you can contact safely. By giving each other one-word clues that can point to multiple words on the table, you try to find all the agents before your turns run out.