Season 7, Episode 21 (February 14, 1979). Written by Dennis Koenig; Directed by Alan Alda.
The episode that finally put Colonel Flagg's name in the title. The MASH4077TV episode guide summarizes it simply: "Colonel Flagg comes for a visit, this time with the notion that Hawkeye is commiserating with the enemy" [1]. (As with "The Abduction of Margaret Houlihan," the season/episode numbering differs between sources—MASH4077TV's broadcast-order guide places this in Season 7, Episode 21, while some other references put it in Season 8.)
After Radar leaves for a stateside assignment in "Good-bye, Radar" (S7E04 in the same numbering), a young replacement clerk named Spicer (Joshua Cadman) arrives at the 4077th. When Spicer's radio breaks, the 4077th is cut off from the outside world for a day—and Flagg (Edward Winter) shows up to investigate why the camp has gone dark [2].
Without Radar to anticipate, deflect, and generally neutralize his every move, the camp is uniquely vulnerable to Flagg's interrogations. He badgers Hawkeye, B.J., Winchester, and Potter in turn, looking for a spy behind every blackout. The episode is in many ways a love letter to Radar by way of contrast: the camp has to muddle through the way the rest of the world does when nobody is reading their mind [2].
The title is a riff on the Civil War song "Rally 'Round the Flag," repurposed to send up Flagg's compulsive self-importance. It's the last major Flagg outing in the series, and arguably the most pointed—having removed the 4077th's secret weapon (Radar), the writers let Flagg have a much bigger field to play in. Winter would go on to reprise the role in the AfterMASH episode "Trials" [3].
[1] M*A*S*H Episode Guide - MASH4077TV [2] Rally 'Round the Flagg, Boys - MASH Fandom [3] List of M*A*S*H characters - Wikipedia