Exposition (Initial Situation)
Hume isn't impressed with what some other philosophers have had to say about morality. Their theories have, in Hume's eyes, been vague, confusing, and just plain wrong. For him, it's obvious that morality isn't just based on pure reason, but on sentiment, too. His aim in the Enquiry is to convince readers that this is the case (kind of like the contestants on The Apprentice) and to show those other philosophers who's boss.
Rising Action (Conflict, Complication)
While some other philosophers seem out of touch with reality, Hume explores two qualities that play a big role in our lives and the good of society: benevolence and justice. Benevolence is obviously agreeable but, with justice, things get more complex. Hume's not daunted, though, explaining that systems of justice have been created because a) they're necessary and b) they help keep everything running smoothly.
Climax (Crisis, Turning Point)
Selfishness: Yay or Nay?
Unlike some folks, Hume doesn't give selfishness the cold shoulder. His view is that there's no reason why self-interest can't go alongside concern for others. Some philosophers claim that benevolence is just a disguise for selfishness but Hume turns the tables on them. If we're totally selfish then why can we feel emotions about stuff that doesn't concern us? And why does a parent spend time and energy caring for a sick child when there's nothing in it for them? Yep, these guys just got owned.
Hume and the Last Crusade
Having established that human beings have a sense of natural sympathy (rather than just being selfish), Hume emphasizes that usefulness is another key factor in making something a virtue; so is agreeableness. Forget so-called virtues like self-denial and solitude—Hume sees them as neither useful nor agreeable. He recognizes that folks sometimes feel greed/vanity and give into temptation, but he's convinced that we have a natural sense of humanity. You've gotta admire this guy's optimism.
Summing up his key arguments, Hume states that morality is based on both reason and sentiment. Also, we're not totally selfish but have a natural sympathy for other human beings. Not all virtues are natural and instinctive: justice systems were created to serve a need. Anyhow, usefulness and agreeableness are what it's all about. There's no need to get nitpicky over labels.