Plot (Spoiler)

With Quagmire unable to drink due to medication, the guys take their families to an Escape Room with Brian using Stewie to solve the problems then taking credit. Brian later shows Stewie a video showing Chris solving a Rubik’s Cube, and explains he was smart as a baby but lost his intelligence when he hit puberty as will Stewie. Stewie creates a machine to alter his DNA to prevent that, but it instead leaves him as an ordinary baby. Unable to cope, Brian tries to undo the process which goes badly wrong like Stewie getting rearranged and turning into a monster. Seeking guidance, Brian tries to watch Splice, but finds Peter has taped over it. Out of options, Brian time-travels back to just before Stewie finished the machine and explains it was a prank by playing Chris’s video backwards, showing him eating the Rubik’s Cube. As revenge, Stewie orders Brian to drown his future self to fix the timeline.