Philistines wage war against the Israelites.
Well, in this chapter David and his 600 Israelites want to join the Philistines. After all, they've been living in their lands for a few years now.
During a pass-in-review of all the lords of Philistine and their armies before their King Achish (Achish might just be a Philistine title for a king, too); David and his 600 Israelites also march past the king. However, the Philistine lords ask, "Why is this Hebrew among us? Isn't this David, the man whom they sing of: "Saul has killed his thousands; David his tens of thousands"?
"Don't let him go to battle with us, unless he is an adversary," said the Philistine lords. By the way, the word "adversary" in Hebrew, which this story was translated from, is "Satan". The Philistines basically called David "Satan", which in context wasn't that bad of a thing, ironically.
I guess it's puzzling to see a Hebrew join their nation and so gleefully go to war against his own people.
After all this protest from the lords of Philistines, the King Achish capitulates and informs David that he can't go to battle with them against the Israelites.
David, disappointed, took his 600 men and left while the Philistines departed to meet the Israelites in battle at a place called Jezreel.