David is tired of being on the run so he goes and does the least obvious thing: make an alliance with the Philistine king, Achish.
This is where things get interesting. Instead of playing Robin Hood, David decides to play the part of a traitor, all so he can get the throne from Saul. It's all very clever. Kudos to David's brain (1-7).
Achish, for good reason, doesn't trust David.
After letting him set up shop in Ziklag, Achish tells David he must prove his loyalty to the Philistines. David does this by raiding the Geshurites, the Girzites, and the Amalekites.
He killed everyone, but he brought animals and clothing to Achish.
When Achish asked him where he had been raiding, he lied to him by telling him that he had raided Israel and its allies. David continued to tell Achish this lie.
Achish believed that he could trust David because he thought that the Israelites would hate him for attacking them. Of course, no one could catch David in this lie because David killed everybody. We told you that David is an outlaw who looks like he's running his own ancient mafia (8-12).