The “Ring of Gyges” begins with a challenge put forth by Glaucon - he wants Socrates to defend the just life and he wants the defense to show that justice is intrinsically preferable to injustice. For the sake of the argument, Glaucon proposes to present a defense of injustice.
Glaucon begins by asserting that people find it desirable or good to inflict wrongdoings on others but these wrongdoers regarded being on the receiving end of misdeeds as undesirable. When people have been on both ends of misdeeds (giving and receiving), they quickly realize that the pains of being a victim far outweigh the benefits of being the victimizer. To avoid being victims, people come together and forge agreements and dub these agreements with the name “justice.”
Glaucon makes it clear that people do not enter into the agreement that gives rise to justice willingly and that this situation is not regarded as the best. He regards justice as a compromise between what is most desirable to the individual (doing misdeeds with impunity) and what is the most undesirable for the individual (being a hapless victim). He further concludes that people accept justice because they are weak and that a person with the power to successfully carry out misdeeds would be a fool not to do so.
In support of his claims that no one is willingly a follower of justice and that anyone who was free to be unjust would be unjust Glaucon tells the tale of the ring of Gyges. In this tale the shepherd Gyges finds a magical ring of invisibility within a strange bronze horse that has been exposed by an earthquake. Using the power of the ring, he seduces the queen and, with her help, murders the king and takes control of the realm.
Given his tale, Glaucon concludes that if identical rings were given to a just man and an unjust man, then both men would act unjustly. This proves, to his satisfaction, that people act justly only under compulsion. By nature, he claims, all living beings desire more than what they are actually due. Despite this, he does consider the possibility that someone might decline to use the ring to perform misdeeds. While such a person would be praised to her face, she would be regarded as a great fool for not using the power in her possession.
Glaucon finishes his case by presenting the details of his challenge. In this challenge the perfectly unjust man is to be squared off against the just man. The unjust man must be the very pinnacle of injustice and must have all that he needs to be unjust and carry out his misdeeds effectively and secretly. To this end he is, for the sake of the argument, given great skill in the use of both persuasion and force and is equipped with various virtues such as bravery and strength. He is further to be blessed with wealth, companions, and an unblemished (though false) reputation for justice. In short, though he is truly a master of injustice he is regarded by all as a just man.
In stark contrast, the just man, while truly just, is stripped of everything but his justice and his life. He is burdened with a reputation for being unjust, despite his true nobility. After all, as Glaucon points out, the just man must be properly tested to see whether he acts justly for the sake of justice or merely for the sake of the reputation and all that goes with it.
Given this setup, it must be determined which man is happier – the just man or the unjust man.