Then the cast of characters meet at the Belgravia home, Duke's Gate, London SW1, of the eccentric Lord Pastern & Bagott, his long-suffering French wife Lady Cécile and her daughter by a previous marriage Félicité (Fée) de Suze.

The plot is driven by the mysteriously owned popular magazine Harmony with an agony column signed by the anonymous GPF ('Guide, Philosopher, Friend') and the enthusiasm of Pastern to play percussion in Breezy Bellairs swing band, resident at the Metronome nightclub run by Caesar Bonn.

The novelty numbers, including 'The Peanut Vendor', 'The Umbrella Man' and 'Hot Guy, Hot Gunner' are to culminate with Rivera being 'shot dead' by a dummy firearm from Pastern, then carried off with a wreath and a funeral march played jazz-style.

“Chief Inspector Alleyn, who actually witnesses the crime, takes half the book to solve it, and I can only urge readers, while he is doing so, to get on with their skimming.”[5] The Scotsman liked Swing Brother Swing, and remarked on her writing: “The plot is clever but Miss Marsh’s virtuosity in building up suspense is the thing.”[4] The Illustrated London News gave it a mixed review, stating that, after the murder, "the falling-off begins; there is too much unrelieved detection, and too much Alleyn.

“Although [Marsh] attempts to give the novel a contemporary feel,' writes Dr Lewis, 'with references to food rationing, six-year-old dresses and the "exhausted aftermath" of the war, it is clearly based on her pre-war memories of London, where she danced at nightclubs like "The Metronome" with the Rhodes.

The plot is weak and trivial... and in comparison to the originality of her New Zealand-based novels, Swing, Brother, Swing appears a retrograde step.”[7] Marsh's later biographer Joanne Drayton[8] is equally unenthusiastic about 'a formulaic book' in which Marsh 'fell back on what she knew to produce something that bordered on the hackneyed', referring to an aristocratic ambience, characters and themes that are 'anachronistic comic cliché straight out of Ngaio's property box of 1930s characters', seeing 'no place in post-war Britain for [ Lord ] Pastern's hedonism...

Drayton draws a parallel between Marsh's fictional Breezy Bellairs Boys and the real American band, Spike Jones & His City Slickers, whose novelty numbers and zany spoof versions of ballads were popular in the 1940s.