Act I

Morning, at the airfield

King Xerxes admires the new Spitfire, which he hopes will transform his continental campaign. Overhearing Romilda, who entertains the wounded as part of her volunteer nursing, he tells his brother, the daring pilot Arsamenes, to bring the lovely singer to him. Arsamenes (himself in love with Romilda) declines to be his brother’s messenger, and complains of the unsuitability of a ‘common’ girl for a king.

Arsamenes warns Romilda of Xerxes’ new passion. Her sister, Atalanta – who loves Arsamenes herself – is intrigued. Xerxes proclaims his fascination, but when Romilda does not reciprocate he blames Arsamenes. Amastris, the foreign princess to whom Xerxes is promised, has landed at the base to find out why all lines to Xerxes have gone dead. She disguises herself as one of the foreign pilots. She overhears the king explaining to the Spitfire’s inventor, Ariodates, that his daughter Romilda will marry someone from the nobility; she then hears Xerxes’ private expostulations of love for Romilda.

Night at the airfield

Amastris vows to avenge her slighted love. Arsamenes, confined to punishment duty, sends his bat man Elviro to Romilda with a letter. Atalanta tries vainly to convince Romilda that Arsamenes is unfaithful.

Act II

The next morning

Disguised as a black marketer (less of a disguise than a part time job), Elviro meets Amastris and describes Xerxes’ new love; then he delivers Arsamenes’ letter to Atalanta, who assures him that she will pass it on to Romilda. However, she passes it straight to Xerxes, saying that it was written to her, and that Arsamenes’ professed love for Romilda is only a cover-up for his real attachment to her.

Xerxes passes this evidence to Romilda; she is hurt, but she still rebuffs the king. Elviro repeats to Arsamenes the lie told him by Atalanta – that Romilda has been won over by the king. The king then acknowledges to Arsamenes that he has discovered his secret devotion to Atalanta, but Arsamenes stakes his claim for Romilda.

Amastris interrupts another attempt on Xerxes’ part to seduce Romilda; only Romilda’s intervention rescues Amastris from the king’s fit of temper.

Act III

Later that day

Arsamenes and Romilda confront Atalanta, who admits her deceit. Arsamenes overhears Xerxes’ renewed, desperate bid for Romilda’s love; after she fearfully accedes, Arsamenes turns on her. Ariodates presents Xerxes with the latest advance in the war effort: the technology for aerial bombardment. Jubilant Xerxes confirms to Ariodates that Romilda will marry into the royal family. He does not reveal that he is to be the groom because he knows there will be an immediate reaction to his decision to marry a commoner – the woman he loves. Ariodates blithely assumes the intended royal groom is Arsamenes, as he is her acknowledged suitor.

Romilda tries to dim her own attractiveness – and suitability – by suggesting to Xerxes that she is not quite a virgin. Xerxes responds by condemning his brother to death. Arsamenes does not heed Romilda’s warning message, transmitted to him by Amastris; he assumes Romilda is trying to shake him off, in favour of his better-placed brother.

Ariodates interrupts a heated argument between Arsamenes and Romilda, and informs them that Xerxes has arranged their marriage. Xerxes arrives to marry Romilda, and finds her already wed. His fury is reflected by a sudden air raid. He calls for his brother to kill Romilda, inasmuch as she has her promise of love to him; Amastris’ timely exposure of his own breach of promise renders her attractive to Xerxes, and her forgiveness ensures this more ‘suitable’ alliance.