Natural Law Ethics (NLE) is an extension of Aristotle's virtue ethics. Aristotle's initial ethical theory has been criticized by some, not for its emphasis on moderation (courage, for example, is the Aristotelian Mean between cowardice and rashness), but for its alleged inability to explain specifically what actions qualify as courageous, temperate, just, and prudent: the four cardinal virtues.
What NLE does is expand upon virtue ethics by stating that the good of a person involves mastering these virtues by the following: act to ensure that the secondary purpose of a human function does not supersede its primary purpose. Yes, NLE presupposes teleology (the study of purpose, telos being purpose itself). I don't wish to defend NLE at this time, but I will provide an example of it.
According to NLE, it is morally wrong to use alcohol to the point where secondary functions of the liver and brain - metabolizing alcohol and drinking to excess so that the person no longer thinks clearly, respectively - supersede those organs' primary functions. Once this is done, then the alcohol has been abused. This is one of the reasons NLE gives for acknowledging that alcohol abuse is morally wrong.
Now, what about the objection I had in mind? Let's forget about alcohol for a moment. Imagine you are a German citizen during WWII, hiding a Jewish family in your home. A small group of Nazis comes to your door and asks if there are any Jews in your home? What is your answer based on NLE? On the one hand, lying is wrong on NLE because it frustrates the primary purpose of a person's rationality. On the other hand, giving the family over to the Nazis, knowing they will be sent to a concentration camp, is also opposed to the NLE, since we are morally obligated to protect the innocent.
Is this a sound objection to NLE? Actually, and I rarely use terms of derision, I find such an objection (though common) to be incredibly sophomoric. What's the answer to the question in the above paragraph? The answer is to lie and protect the lives of an innocent family. Why? The reason is that on NLE, some priorities are more important than others. This doesn't condone lying, but if put in this situation, the lives of innocents are more important.