A variant on the legend in chapter 24. In this one, Saul hears that David is hiding in the wilderness and gathers his 3000 men to attack him. David scouts out Saul's position and asks two of his lieutenants to help him.
David sneaks into Saul's tent that night with one of them, who encourages him to kill the sleeping king. David refuses, because it's god's call. They do, however, take his spear and a pitcher of water.
The next morning, David calls out to Abner, captain of Saul's guard. Huh, so Lil Abner has biblical roots. Anyway, David taunts Abner with the spear and jug until Saul figures out who's shouting. David asks him why he's trying to kill him. Duh, he's nuts. Saul says he's sorry and agrees not to try and kill David anymore. Yeah, that'll last.
David decides that he can't trust Saul and joins the Philistines as a marauder. Saul finds out where he is and leaves off chasing him for a bit. He asks the king for a city and gets the colourfully-named Ziklag, which apparently still belongs to the kings of Judah. He stays there for 16 months, staging raids on the surrounding tribes. He kills a lot of people and keeps a lot of animals. The king believes that the Israelites must hate David by now, and he'll be his servant forever.