This episode first aired six years ago. In 2005, what has recently become known as "the R word" was I guess not such a big deal. Would they do a show prominently featuring the term "retarded" today? It's hard to say - after all, we're talking about the show that last year included a hilarious musical number "Downs Syndrome Gal."
We open with a series of Family Game Nights including Twister, Cleveland's civil rights board game, and a game of Trivial Pursuit in which Peter struggles with the question, "What color is a fire truck?" Flush with his success (Lois swapped in the kid's version questions for Peter), he annoys everyone until Brian finally challenges him to get officially tested as a genius.
Predictably, Peter fails the test. In fact, based on his test results, he is officially certified as "mentally retarded." After a period of adjustment, Peter suddenly discovers that he can get away with anything, using "I don't know any better" as an excuse. This includes beating people up at church, busting into the ladies' room and kicking open the stalls, commandeering the intercom at a fast food restaurant and saying "testicles" over the loudspeaker, and hiring a motivational coach named Vern (voiced in a hilarious cameo by LeVar Burton).
Peter accidentally knocks boiling oil all over Lois, who has to recuperate in the hospital. In her absence, Peter is certified mentally unfit to take care of their children, and Child Protective Services places Meg, Chris, and Stewie in foster care with Cleveland.
(Not to be confused with the time CPS took Stewie away, mistakenly believing that Meg was a single mother who had been feeding him crack and pancakes. In that episode, Stewie ends up in foster care with a multicultural collection of other babies, and leverages each baby's nationalism to turn them all against each other.)
In the end, Lois heals, and they get the kids back, and everything is okay. Kind of a boring ending, but some pretty funny stuff along the way.

Best Cut-Away
A tie between Brian's "More stupid than that time you locked your keys out of the car?" and Stewie's iPod commercial (to Scandal's 1984 hit "The Warrior").

Best Moments
"A solitary killer, the fire truck stalks its prey… the fire truck can consume eight times its body weight. The ambulances will have to wait their turn."
Peter: "Well Lois, since you asked, I find this meatloaf to be shallow and pedantic."
Stewie to a prostitute: "So, is there any tread left on the tires at all? Or at this point would it just be like throwing a hot dog down a hallway?"