Peter always loses many different games so Lois switches his Trivia Pursuit questions with the kids version and Peter ends up winning. He can't stop bragging about winning so Brian tells him to take a test that will tell him if he is a genius or not. Peter takes the test and is declared legally retarded.
Peter learns to use this condition to his advantage after he hits Tom Tucker with his car. Peter does things like walk into the ladies' room. Peter learns his lesson when he accidentally throws hot grease all over Lois and burns her and social services takes away his kids.
Lois brings the kids back in the end and Peter stops abusing his condition.