Sam (Jared Padalecki) and Dean Winchester (Jensen Ackles) are doing research at the house of Bobby Singer (Jim Beaver) when the rogue angel Balthazar (Sebastian Roché) appears.
He says that the archangel Raphael is aiming to kill all the allies of the angel Castiel (Misha Collins), and so he gives them a key and sends them to an alternate reality to keep them out of harm's way.
When they appear in the alternate reality, Sam and Dean find themselves on the set of Supernatural—a fantasy horror television series that is being filmed in Vancouver, Canada.
In this dimension, Sam and Dean are actors known as "Jared Padalecki" and "Jensen Ackles", and they star in the aforementioned show, which follows the adventures of the fictitious Winchester brothers.
Discouraged that the ingredients for Balthazar's spell cannot be located on the set of Bobby's house, Sam and Dean have the actors' driver, Clif Kosterman (Phil Hayes), take them to Jared's place, which turns out to be a large ostentatious mansion.
After Genevieve heads out to a charity function for the International Otter Adoption Fund, Sam and Dean go online and purchase bonafide saints' bones for rush delivery using Jared's credit cards.
Before they are able to work the spell, however, the show's director Bob Singer (portrayed in the episode by Brian Doyle-Murray) forces them to act in a scene, which ends badly after many takes.
Just then, Balthazar arrives and reveals that the key which Sam and Dean had the entire time was a fake and that the sojourn through the alternate reality was a diversion to throw off Raphael and his minions.
[1] Diana Steenbergen of IGN gave "The French Mistake" a 9.5 score out of 10 and applauded the show writers for taking "an insane idea and turn it into gold".
[6] In her book Supernatural: A History of Television's Unearthly Road Trip (2021), Erin Giannini ranked "The French Mistake" at number 16 on a list of "must-see" episodes.