This is a paraphrase of story that can be found in The Book of JudgesBuy Teddy Bear Cannibal Massacre
The Book of Judges is an entire litany of Israel falling away from monotheism, Israel being conquered, someone coming to save Israel from its neighbors.In the case of Ehud, the Moabites are the conquerers. As required, the Israelites cry out, Ehud takes center stage. Moabites die, there's peace.
However, the central portion of the Ehud story is what distinguishes him. Going to the Moabite king as an emmisary, Ehud and fellows work out a peace treaty. After they leave, Ehud comes back and stabs the king, but since the king is so fat, Ehud can barely get the knife out. This is one of two fat jokes in the Bible (the other one being the unfortunate fate of Eli the Priest in 1 Samual) and after this, Ehud drags the king into the restroom and runs off.
Meanwhile the advisors, noticing that the king and Ehud have been in conference for a little too long go to see how the king is doing. They notice that the rest room is locked so they assume that the king is "cooling his feet" which is the ancient equivalent of "draining the lizard"(I've seen very few Bibles that explain Judges 3:24, it just sounds confusing most of the time.) By the time they get the courage to interrupt the king on his throne, Ehud is halfway to the camps and the troops are in ready position.
The Moabites really don't bother the Israelites much after that.