Miss Lessing, the mistress in charge of Middle House at St. Helen's School, leaves at a fortnight's notice to be married when her fiancé has unexpected leave from New Zealand. The Middles are very upset, and determine to give her successor a difficult time. Unknown to them (and to be kept secret at her own request), the new Housemistress is an Old Girl of the School. She arrives over the half term weekend, to be greeted with indifference and even rudeness by several of her charges.
The Middles take their cue from "The Octave", chief among whom is Barbara Allen. However, their campaign is shaken on the very first evening when one of their number, Mollie Robson, climbs an unsafe tree for a dare: the despised newcomer performs a nick-of-time rescue which compels the unwilling admiration of the watching girls. Despite this, Barbara and her fellow Middles pursue their efforts to make Miss Oswald realise that she is not wanted.
In an attempt to enliven their free time and flout Miss Oswald's authority, the Middles decide to put on a play in one of the dormitories after lights ou - complete with curtains, programmes, stage lighting - of sorts - and invitations to the audience. They write a stage version of the ballad "Barbara Allen", with their own Barbara as the heroine, and sundry additional characters thought up to provide enough parts for all those involved.
The performance takes place without being discovered by anyone in authority, but shortly after the actresses are back in bed, fire sweeps through Middle House. The building is evacuated, but Barbara is the last to use the fire escape and is overcome by smoke before she can descend. Miss Oswald once again comes to the rescue, although she is quite seriously injured in the process. It is discovered that the fire was caused by a discarded candle from the Middles' "stage lighting".
When Barbara is well enough for visitors, she hears how, and by whom, she was rescued from the burning dormitory. Unwilling to believe it, she asks to speak to Miss Blake, the Headmistress, and learns that other members of her family also owe their lives to Miss Oswald. The Head assures Barbara that "next term everything will be all right."
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