Hello! In a nutshell, MacIntyre's 'After Virtue' is basically about the main problem inherent in Western ethics: its lack of a central, coherent framework to explain its main precepts and values. He states that Enlightenment philosophers like Kant, Kierkegaard and Hume, for instance, failed to express their philosophies in a morally coherent manner. One of the failures of the Enlightenment is the disavowal of the Aristotelian concept of teleology (the doctrine that final causes exist and that every human being has a reason for existing; the design inherent in every living thing speaks of its purpose in life). Since the Renaissance rejected this teleological aspect of its moral framework, Western ethics became mired in the concept of emotivism (this is the doctrine that what one feels/emotes decides one's moral values).
MacIntyre acknowledges Nietzsche's reason for why Aristotelian teleology was so rejected: its incoherence contributed to no small amount of confusion and frustration. This is why Nietzsche's solution of the Ubermensch (each man determining his own moral values for himself, apart from social and religious interference) is his cry against the moral incoherence of teleology; it is also Nietzche's solution to total nihilism. MacIntyre's main thesis is that we have to understand where we came from to understand who we are and that Western ethics (teleology) has to better explain its central tenets; without it, our world of ethics becomes just a seemingly arbitrary set of rules with no context and no relevance to life.
See the links below for more detail. Hope this helps!