After many years of being estranged from his family, Andrew Undershaft, a wealthy industrialist who specializes in the manufacture of gunpowder and weapons, returns home to meet his three children. His wife, the Lady Britomart, has her own plan to convince him to make his son Stephen the family heir, despite the Undershaft family tradition of appointing a foundling to inherit the family fortune. Caught in the middle of this is the title character, Undershaft's rebellious older daughter and a major in the Salvation Army, who tries to get everyone to see the value of her work.
As the agendas and philosophies of the different family members collide, the question is raised: who among them is truly in the right?
Tropes present in this work:
- Arms Dealer: Undershaft runs a business empire based on this.
- Bread and Circuses: According to Undershaft, the Salvation Army embodies this trope, hence why the wealthy continue to fund it.
- Catchphrase: "Oh I say!" for Lomax.
- Decoy Protagonist: The title might point to Barbara, but the real protagonist is her father Undershaft.
- Embarrassing Nickname: Cholly and Dolly for (Charles) Lomax and (Adolphus) Cusins.
- Funetik Aksent: Price and Rummy, and other poor people.
- Gambit Pileup: Britomart wants Stephen to be made heir, Undershaft just wants to get to know his family (at first), Barbara wants the Salvation Army to be recognized, Cusins is just in love with Barbara, and Stephen (semi-secretly) wants to go into politics instead of being his mother's puppet.
- Gentleman Snarker: Undershaft is fairly polite and formal, and very snarky.
- Grande Dame: Lady Britomart.
- Honor Before Reason: Barbara's decision to leave the Salvation Army is because they accept the money of rich men like her father, despite them being supposedly the source of the problems the Army exists to fight against. Her assistant, Mrs. Baines, calls her out on this to no avail.
- I Am the Noun: Undershaft's "I am the government of your country" speech.
- Laser-Guided Karma: Bill, who slaps Rummy, gets his comeuppance by having the new boyfriend/husband (the play is unspecific) of his ex-wife sit on his head and then pray for him.
- Momma's Boy: Stephen. After having met his father, he finally stands up to Britomart.
- The Nicknamer: Undershaft calls Cusins "Euripides". Cusins counters by calling Undershaft "Machiavelli".
- Orphan's Ordeal: Discussed. Undershaft indicates this is why the family tradition exists, as someone who has not undergone this is not fit to run a business empire like his.
- Seven Deadly Sins: Played with; Undershaft defines the seven as "food, clothing, firing, rent, taxes, respectability, and children".
- Take That!: Undershaft saying "He [Stephen] knows nothing; and he thinks he knows everything. That points clearly to a political career."
- 20 Minutes into the Future: Although set in 1906, a year after it was written, Undershaft mentions the military success of his "aerial battleships" despite the fact that very few zeppelins were operational at the time and the first bombing by one was not conducted until 1914.
- "Well Done, Daughter!" Girl: Barbara already got rejected by her mother, so the first thing she does when meeting her father is try to convert him to the Salvation Army's cause.