Which two-part M*A*S*H episode introduced B.J. Hunnicutt?

The two-part M*A*S*H episode that introduced B.J. Hunnicutt was “Welcome to Korea.”

It opens the show’s fourth season and follows Hawkeye Pierce and Margaret Houlihan as they return to the 4077th after learning that Trapper John McIntyre and Henry Blake have gone home. The unit is also adjusting to Colonel Sherman Potter, who replaces Blake as commanding officer. During the episode, Captain B.J. Hunnicutt arrives as the new surgeon and becomes Hawkeye’s partner in the operating room and a central member of the ensemble.

B.J. was designed to fill Trapper’s medical and comic role while giving the series a different personality. Unlike the unmarried, mischievous Trapper, B.J. is married and generally more openly affectionate toward his family. His initials are not an abbreviation for a longer name: the show explains that “B.J.” comes from his parents’ names, Bea and Jay.

The episode is sometimes confused with “Fade Out, Fade In,” which introduces Major Charles Winchester in Season 6. “Welcome to Korea” instead marks the transition from the original post-Season-3 cast to the Potter-and-B.J. era.