Who will guard the guardians?

The movie The Watchmen which is currently in theaters doesn't appeal to me, but one of the basic themes is quite interesting. Who will watch the watchmen?

The question was posed by Plato in his book The Republic.

In the perfect society as described by Socrates, the guardian class is to protect the city. The question is put to Socrates, "Who will guard the guardians?" or, "Who will protect us against the protectors?"
They will guard themselves against themselves.
Plato says we must tell the guardians a "noble lie", convincing them that they are better than those they serve, and it's their responsibility to guard and protect people lower than themselves. We must instill in them a distaste for power or privilege and convince them serve because it is the right thing to do.

This also reminds me of the classic "a watchmaker needs a watchmaker-maker" paradox about how if the universe needed a creator then by that same logic that creator needed a creator of its own.
and it goes on and on.
If everything must have a cause, then God must have a cause. If there can be anything without a cause, it may just as well be the universe as God.