He told Crave Online that the hour-long episode will feature a murder mystery plot.
"Our murder mystery... is probably one of my favourite shows that we've ever done," claimed MacFarlane.
"The story is constructed so effectively... as a mystery. I think people are going to be laughing but at the same time they're actually going to be wondering who the hell we're going to kill off next, and who the murderer is."
He promised that fans will not be disappointed by the reveal of the killer's identity.
"It turns out that it is somebody fairly significant," he teased.
The episode - featuring guest star James Woods - will air on September 26 on Fox.