Season 5, Episode 6 (October 26, 1976). Written by Story by Arthur Broomfield & Dennis Koenig, Teleplay by Dennis Koenig; Directed by Alan Alda.
Margaret Houlihan (Loretta Swit) goes missing from the 4077th, and Colonel Flagg (Edward Winter) is dispatched to investigate. As the MASH4077TV episode guide summarizes it: "Frank thinks she has been captured by the North Koreans. As it turns out, she was only helping a South Korean woman give birth, and everyone is relieved when she returns" [1]. (Note: this episode's season/episode numbering is inconsistent across reference sources—Wikipedia places it in Season 6, while the MASH4077TV broadcast-order guide places it in Season 5, Episode 6. The October 26, 1976 air date and Frank Burns-era cast are the same in both.)
The episode pits Flagg's instincts directly against Hawkeye, B.J., and Colonel Potter, who are all that stand between Margaret and Flagg's habit of making everything worse. It gives Loretta Swit a rare showcase in which Margaret is genuinely the central figure rather than the stock "Hot Lips" caricature [2].
The Flagg of this episode is operating at full menace: convinced that Margaret has been taken by communists, ready to escalate the search into a counter-intelligence operation, and treating the 4077th as a suspect pool rather than a rescue team. The camp's repeated frustration with him is the closest the early Frank Burns era comes to a sustained Flagg storyline [1].
[1] M*A*S*H Episode Guide - MASH4077TV [2] The Abduction of Margaret Houlihan - Wikipedia [3] List of M*A*S*H characters - Wikipedia