Bill and Ben have been arrested and are being held in custody. They were found fighting after a failed burglary, but the police have a problem. There is no real evidence against either Bill or Ben that they were involved in the attempted burglary. It is entirely circumstantial that they were close to the scene when fighting, although it is probable that the failure to effect the robbery caused them to fight.
So the police concoct a "cunning plan".
They offer both Bill and Ben the following plea bargain: If you testify that your accomplice was involved in the burglary and he remains silent, you will go free and he will go to prison for ten years; if you both remain silent, you will both go to jail for one year for fighting; if you both testify, you will both be incarcerated for five years.
Bill and Ben have no way to communicate, and so no way to agree a plan.
What should they do?
This is quite a well known puzzle, known as the Prisoners' Dilemma, and there is no real correct answer.
The best collective course of action is for both Bill and Ben to stay silent. In this way they both get a year inside and then walk away.
Individually keeping silent has a big risk, because if your accomplice testifies against you a ten-year stretch awaits you, while he walks free.
So the insurance of talking means you could walk free, and in the worst case your sentence would be reduced to five years.
But the two prisoners are still better off keeping quiet, if they can trust the other.