Plato explicitly states in the Republic that the ideal city described should not be interpreted as a model for an actual city, but rather as a way of discussing the soul by examining it through a macrocosmical analogy. Plato's actual recommendations for a system of practical governance are found in a later dialogue, the Laws. However, given substantial biographical evidence that Plato himself avoided practical politics, except for a rather unsuccessful attempt to advise leaders in Syracuse, it is probably better to think of Platonic philosophy as a system for understanding the soul than as a system for organizing the state.
The Allegory of the Cave suggests that the philosopher, who contemplates the true forms (the objects outside the cave) should return to the cave to advise those inside who have seen only shadows. The philosopher, though, is not intended as a ruler, for the practical activity of ruling is incompatible with the philosophic life. Instead, the ideal ruler should be one trained and advised by philosophers.