The first big change is that Duet takes the basic wordplay of Codenames and packages it in a totally cooperative experience. Rather than two teams competing to be the most clever, everyone must now work together to contact all of the agents.
As before, you have a grid of word cards and a key card that tells you which words you’re looking for, which are neutral, and which are deadly assassins. But now the key card is double sided. So each side of the table is trying to get the other side to guess words. Of course, you only have nine clues to guess 15 words. So you still have to strive for cleverness.
But that cooperative element means that the game works well with as few as two players. In other versions of the game, you really need at least four to get a good experience. But now, you can play as a couple. And it still works for larger groups. In fact, with half the table able to see each side of the clue card, you get lots of whispers and multiple suggestions about what the next clue should be.