Season 3, Episode 10 (58th overall), first aired November 19, 1974. Written by Larry Gelbart; directed by Hy Averback [1][2].

When a credible threat of a North Korean paratroop drop reaches the 4077th, the nurses are evacuated to a field hospital. The surgeons are left with unqualified and often incompetent replacement staff, and with no women to flirt with. Hawkeye and Trapper fill the time with the usual mix of mischief and romantic hopefuls [1][3].

Captain Calvin Spalding, the wandering minstrel played by Loudon Wainwright III, returns for his second appearance. He sings in the Swamp with Hawkeye and Trapper providing backing vocals, and later performs a maudlin song about the nurses at the O-Club, where Klinger has brought him his guitar. Wainwright wrote both songs [1][3].

Before the nurses leave, Hawkeye gives Margaret a long goodbye kiss while Frank looks on in dismay. Frank gives Margaret an envelope of his hair clippings, which he says were meant for his mother. Frank later calls Margaret at the field hospital in a jealous rage. Hawkeye and Trapper are listening in on the other line, and when Frank chases them outside he falls into a foxhole he dug for the expected paratroop assault. The captains then park a jeep over the foxhole to trap him [1].

The episode's B-plot involves a reel of film hidden under Frank's bunk: his own wedding to Louise. The camp gathers in Henry's office to watch it and mock Frank's nervous, clumsy wedding day [1][3].

The air raid turns out to be Five O'Clock Charlie dropping surrender leaflets. With no attack coming, the nurses return, and Spalding leads a sing-along through the camp. That epilogue is usually cut from syndicated broadcasts [1][3].

[1] There Is Nothing Like a Nurse - MASH Fandom [2] List of MAS*H episodes - Wikipedia [3] MAS*H Episode Guide - MASH4077TV