- Act I: Lady Britomart's house in Wilton Crescent
- Act II: The Salvation Army shelter in West Ham
- Act III: Lady Britomart's house, later at the Undershaft munitions works in Perivale St Andrews
An Officer of The Salvation Army, Major Barbara Undershaft, becomes disillusioned when her Christian denomination accepts money from an armaments manufacturer (her father) and a whisky distiller. She eventually decides that bringing a message of salvation to people who have plenty will be more fulfilling and genuine than converting the starving in return for bread.
Although Barbara initially regards the Salvation Army's acceptance of Undershaft's money as hypocrisy, Shaw did not intend that it should be thought so by the audience. Shaw wrote a preface for the play's publication, in which he derided the idea that charities should only take money from "morally pure" sources. He points out that donations can always be used for good, whatever their provenance, and he quotes a Salvation Army officer, "they would take money from the devil himself and be only too glad to get it out of his hands and into God's".
Lady Britomart Undershaft, the daughter of a British earl, and her son Stephen discuss a source of income for her grown daughters Sarah, who is engaged to Charles Lomax, and Barbara, who is engaged to Adolphus Cusins (a scholar of Greek literature). Lady Britomart leads Stephen to accept her decision that they must ask her estranged husband, Andrew Undershaft, for financial help. Mr. Undershaft is a successful and wealthy businessman who has made millions of pounds from his munitions factory, which manufactures the world famous Undershaft guns, cannons, torpedoes, submarines and aerial battleships.
When their children were still small, the Undershafts separated; now grown up, the children have not seen their father since, and Lady Britomart has raised them by herself. During their reunion, Undershaft learns that Barbara is a Major in The Salvation Army who works at their shelter in West Ham, east London. Barbara and Mr. Undershaft agree that he will visit Barbara's Army shelter, if she will then visit his munitions factory.
When he visits the shelter, Mr. Undershaft is impressed with Barbara's handling of the various people who seek social services from the Salvation Army: she treats them with patience, firmness, and sincerity. Undershaft and Cusins discuss the question of Barbara's commitment to The Salvation Army, and Undershaft decides he must overcome Barbara's moral horror of his occupation. He declares that he will therefore "buy" the Salvation Army. He makes a sizeable donation, matching another donation from a whisky distiller. Barbara wants the Salvation Army to refuse the money because it comes from the armaments and alcohol industries, but her supervising officer eagerly accepts it. Barbara sadly leaves the shelter in disillusionment.
According to tradition, the heir to the Undershaft fortune must be an orphan who can be groomed to run the factory. Lady Britomart tries to convince Undershaft to bequeath the business to his son Stephen, but he will not. He says that the best way to keep the factory in the family is to find a foundling and marry him to Barbara. Later, Barbara and the rest of her family accompany her father to his munitions factory. They are all impressed by its size and organisation. Cusins declares that he is a foundling, and is thus eligible to inherit the business. Undershaft eventually overcomes Cusins' moral scruples about the nature of the business. Cusins' acceptance makes Barbara more content to marry him, not less, because bringing a message of salvation to the factory workers, rather than to London slum-dwellers, will bring her more fulfilment.
The play was first produced at the Royal Court Theatre in London in 1905 by J.E. Vedrenne and Harley Granville-Barker. Barker also played Cusins, alongside Louis Calvert, Clare Greet, Edmund Gwenn, Oswald Yorke and Annie Russell.
In the summer of 1988, the Chichester Festival Theatre presented a production with Donald Sinden as Andrew Undershaft, Anna Carteret as Barbara and Marc Sinden as Stephen Undershaft, directed by Christopher Morahan.
A production from 26 February to 3 July 2008 at the National Theatre featured Simon Russell Beale as Andrew Undershaft, Hayley Atwell as Barbara, Clare Higgins as Lady Britomart, Paul Ready as Cusins and Jessica Gunning as Sarah Undershaft.
The play first opened on Broadway at the Playhouse Theatre on December 9, 1915. There have been four Broadway revivals, in 1928 at the Guild Theatre, 1956 at the Martin Beck Theatre and then the Morosco Theatre starring Glynis Johns, Cornelia Otis Skinner, Eli Wallach, Burgess Meredith, and Charles Laughton, who also directed, in 1980 at the Circle in the Square Theatre, and 2001 at the American Airlines Theatre, with Cherry Jones in the title role.
Major Barbara was revived in a production at the Abbey Theatre, Dublin from 21 July 2013. Annabelle Comyn directed; Clare Dunne played Barbara, Marty Rea Adolphus Cusins, Paul McGann Andrew Undershaft, Eleanor Methven Lady Britomart. 
In Season 3 of HBO series Girls by Lena Dunham, a production of Major Barbara on Broadway forms the backdrop of lead character Hannah's relationship with her boyfriend Adam.
Lady Britomart Undershaft was modelled on Rosalind Howard, Countess of Carlisle, the mother-in-law of Gilbert Murray, who with his wife Lady Mary served as inspiration for Adolphus Cusins and Barbara Undershaft.
Andrew Undershaft was loosely inspired by a number of figures, including the arms dealer Basil Zaharoff, and German armanents family Krupp. Undershaft's unscrupulous sale of weapons to any and all bidders, as well as his government influence and more pertinently his company's method of succession (to a foundling rather than a son), tie him especially to Krupp steel. Friedrich Alfred Krupp died under scandalous circumstances in 1902 leaving only daughters as heirs. Undershaft shares a name with a Church of England church in the City of London named St Andrew Undershaft; given the City's longstanding status as the financial center of London, the association underscores the play's thematic emphasis on the interpenetration of religion and economics, faith and capital. Undershaft's morality can be compared to the philosophy of Friedrich Nietzsche.[according to whom?]
- "Major Barbara | Abbey Theatre - Amharclann na Mainistreach". Abbeytheatre.ie. Retrieved 2014-02-24.
- Sidney P. Albert, "'In More Ways than One': Major Barbara's Debt to Gilbert Murray," Educational Theatre Journal, Vol. 20, No. 2. (May, 1968), pp. 123–140, and idem, "From Murray's Mother-in-Law to Major Barbara: The Outside Story," SHAW: The Annual of Bernard Shaw Studies 22 (2001), pp. 19–65.
- 1905 Royal Court production on the Global Performing Arts Database
- Major Barbara at the Internet Broadway Database (1915)
- Major Barbara at the Internet Broadway Database (1928 revival)
- Major Barbara at the Internet Broadway Database (1956 revival)
- Major Barbara at the Internet Broadway Database (1980 revival)
- Major Barbara at the Internet Broadway Database (2001 revival)
- Program from Major Barbara at South Coast Repertory