Sentence:  Given the particular payoffs in Prisoner's Dilemma games, it is always rational to defect on the other player as long as the game is <WSD>one-shot</WSD> and not repeated infinitely.  Regardless of what the other player does, you get a higher payoff by defecting than cooperating. In <WSD>one-shot</WSD> games, there is nothing the other can do to punish you for defecting. 
Meaning of the word marked with <WSD> tags:  Unique; occurring only once.