Colonel Flagg is the 4077th's most persistent unwanted guest. Played by character actor Edward Winter (1937–2001) across six episodes of the series, Flagg is a U.S. Army intelligence agent who periodically descends on the 4077th to investigate suspected wrongdoing, security breaches, and anything else his paranoid mind interprets as subversive. He alternately claims to be affiliated with the CIA, the CIC (Counter Intelligence Corps), or the CID (Criminal Investigation Division), and rotates through a dizzying list of aliases—Major Brooks, Captain Louise Klein, Lieutenant Carter, Ensign Troy, and Chaplain Goldberg—almost none of which he can keep straight [1].
Flagg is a parody of the suspicious, by-the-book intelligence officer: convinced that everyone is hiding something, that conspiracies are everywhere, and that the 4077th's regular personnel (particularly Hawkeye, Trapper/B.J., and Radar) are up to no good. Unlike most of the show's heavies, however, Flagg is played almost entirely for comedy—his over-the-top accusations, rotating code names, and self-important bluster made him a fan-favorite recurring villain, even as he was genuinely menacing enough to rough up the 4077th's staff in several episodes [1][2].
Flagg appears in six M*A*S*H episodes spanning most of the series run. Using the broadcast-order numbering from MASH4077TV's episode guide (which differs in some places from Wikipedia's numbering):
In addition, Winter played a similar character named Captain Halloran in "Deal Me Out" (S2E13, December 8, 1973). Most fans of the show—and Wikipedia—consider this Flagg's actual first appearance, with Halloran simply being one of his many aliases. The case is strengthened by a comment Flagg makes to psychiatrist Sidney Freedman in "Quo Vadis, Captain Chandler?"—Freedman also appears in "Deal Me Out," suggesting the two episodes are connected through the same intelligence agent operating under cover [1][2].
Flagg resurfaces one final time in a Hannibal, Missouri courtroom in the AfterMASH episode "Trials," a few years after the Korean War ends.
Whether "Captain Halloran" in "Deal Me Out" is "really" Flagg is a long-running fan debate. The episode is never stated to be a Flagg appearance, and Halloran is far more affable than the Flagg we meet later—smiling, sharing a drink, and telling Sidney Freedman "we played poker once." On the other hand, Winter's casting in the role is hard to explain otherwise, the Freedman connection is unmistakable, and the show is famously casual about maintaining continuity in service of a good joke. The Wikipedia entry on Flagg treats it as settled, but the show itself never confirms it [1].
Winter (June 3, 1937 – March 8, 2001) was a stage-trained character actor best known for his two Tony Award nominations as Best Featured Actor in a Musical—for Cabaret (1966) and Promises, Promises (1968)—before becoming one of television's most recognizable utility heavies. Beyond M*A*S*H, Winter racked up guest spots on Dallas, Lou Grant, Columbo, The Golden Girls, Seinfeld, The A-Team, The Mary Tyler Moore Show, Murder, She Wrote, and Cagney & Lacey, among many others. His film work included The Parallax View (1974), A Change of Seasons (1980), and Porky's II (1983). He died in 2001 of complications from Parkinson's disease, at age 63 [3].
[1] List of M*A*S*H characters - Wikipedia [2] M*A*S*H Episode Guide - MASH4077TV [3] Edward Winter (actor) - Wikipedia