Pack up your troubles in your old kit bag

"Pack up your troubles in your old kit bag" was a song known on 21st century Earth (PROSE: Winner Takes All ^[+]Loading...["Winner Takes All (novel)"]) and to 51st century ex-Time Agent Captain Jack Harkness. Its chorus contained the lyrics "smile, smile, smile". (PROSE: The Deviant Strain ^[+]Loading...["The Deviant Strain (novel)"])

Mrs Pobjoy, one of the humans captured by the Quevvils to carry out their war against the Mantodeans, repeatedly attempted to engage her fellow captives in sing-songs. Shortly before the Ninth Doctor and Rose Tyler arrived to try and rescue them, Mrs Pobjoy was singing a rousing chorus of "Pack up your troubles", but another captive, Robert Watson, couldn't see that the captives had anything to smile about, although he thought that they might if she stopped singing. (PROSE: Winner Takes All ^[+]Loading...["Winner Takes All (novel)"])

Jack was singing a florid rendition of the song while alone in the TARDIS console room when a distress call was picked up originating from the Novrosk Peninsula in Russia, and Jack didn't notice the warning beeps until he was halfway through the next chorus. (PROSE: The Deviant Strain ^[+]Loading...["The Deviant Strain (novel)"])

Behind the scenes

It is a World War I marching song.