A Clear, Simple Explanation for the Impossibility of Infinite Regress in the First Cause Argument

My interpretation of the First Cause argument for the Existence of God attributed to St. Thomas Aquinas and the essence of which perhaps goes back to Aristotle, appears in my mind to be valid provided that we assume that every phenomenon has an explanation (whether known or unknownst to us) and that each effect must have some preceding cause. It is argued, of course, that this chain can not regress backwards infinitely far---therefore, there must be a *First Cause* (or *Prime Mover*), which is God. Can someone either explain or refer me to a source that explains in simple, crystal clear terms, why infinite regress is so *obviously* impossible?