So David and his family flee to Moab, and after David sees to it that his family is taken care of, he returns to the land of his tribe, Judah, now with a small army to protect him.
Saul meanwhile is very unhappy with, well, pretty much everyone and everything. The fact that David is against him, and Jonathan, his own son, is a friend of David has caused him to become paranoid. He gives a speech to his Benjamite soldiers, pointing out that surely David will not give the sort of deference to the tribe of Benjamin that he himself has. He questions their loyalty.
Doeg the Edomite (a foreigner, in case you didn't catch it) speaks up to show his own loyalty, pointing out that he saw David visit the priests in Nob. Saul sends for Ahimelech and demands to know "Why have ye conspired against me, thou and the son of Jesse...?" Ahimelech insists (honestly, by the look of things) that he had no idea David was not on a mission from Saul, but Saul refuses to believe this excuse, and tells his men to kill all of the priests. The men refuse, but Doeg apparently eagerly complies, killing not only the priests, but every person and animal in the city of Nob. The SAB notes this as violent, to which I wholeheartedly agree, and with the "Family Values" icon, the reason for which completely escapes me.
And now to Abiathar the priest. Here, as I mentioned in my previous commentary, is some confusion as to lineage. Is Abiathar the father or the son of Ahimelech. You know, one thing I tried to do was see if this complex sentence (verse 20) could be unraveled to translate differently, such as the idea that "named Abiathar" could be a reference not to the son that escaped, but that it was saying Abithar was another name for Ahitub, who seems to be clearly the father of Ahimelech (at least in this verse). It doesn't seem to be so, especially since the following verse seems to make clear that Abiathar was the escapee, the clear subject of the sentence.
Clearly there is an error somewhere, but as I said, I can't seem to let it go with that, and I've been doing some reading, trying to unravel it as much as I can. First of all, I'm fairly certain the answer is once again with the minority reading, that is, Ahimelech is the father, despite the fact that more verses name him as the son. Why do I think this? Well, firstly because those verses saying Abiathar is the father are ones that were written later, much longer after the events' actual occurrence, so that makes it seem to me that they are slightly more likely to be erroneous. Secondly, and more importantly, there are several verses such as this one that mention Ahimelech as the son of Ahitub, who is (according to 1Sam.14:3) the grandson of Eli, which seems to fit in pretty good culturally and with the timeline. On the other hand, I don't think there is an instance of a single verse that lists anyone else as being the father of Abiathar other than Ahimelech in these two chapters.
Admittedly, there is still some confusion beyond that. I tried to map out the family tree of Eli, but it gets murky after a few generations, partially due to the fact that there are multiple people with the same name (there seem to be two different priests by the name of Zadok, both of whom were contemporaries of David), and the genealogy doesn't seem to be in anywise complete. This may be the key to the answer, but I'll admit this theory, while possible, doesn't seem to be strongly supported: It may be that since Ahimelech was killed in the massacre at Nob, Abiathar may have named his next child after his dead father as a memorial. In such a case, the latter mentions of "Ahimelech the son of Abiathar" would actually be to the grandson of the Ahimelech in these two chapters. I think if this were the case, there would be some mention of the fact that this special repeat naming had taken place (after all, it seems to me it makes for an interesting story) but there is no mention of it, so it remains questionable speculation.
Whoever Abiathar is (and I'll say that despite Jesus referring to him as the high priest, I think there's still possibility that he was not high priest until after the massacre, if that sounds weird, feel free to ask in the comments), David invites him to come with him and his company, as they now share a common enemy in Saul.