Duchess's Marriage
The play begins on a note of dramatic irony as the Duchess pursues Antonio, her steward. Her brothers, however, do not know that the Duchess is married as she swore to them that she would never take another husband. As the tension builds, the stakes of the Duchess's secret marriage increase, foreshadowing the violence that will ensue when her brothers discover her secret.
Bosola
When Bosola realizes his villainy halfway through the play and decides to make amends, the Cardinal is still under the impression that Bosola is working for him. The Cardinal sends Bosola away to murder Antonio, not knowing that Bosola plans to help Antonio and his child escape.
Antonio's Death
In a moment of tragic irony, Bosola – who now sought to free Antonio and lead him and his child by the Duchess to safety – ends up accidentally killing Antonio instead. This plot trajectory is one of the reasons why the play is often described as bleak and despairing: even after Bosola is reformed, innocent people are still murdered.
The Duchess's Autonomy
In another of the play's tragic ironies, the Duchess begins the play with full confidence that she is in control of her own life. She deceives her two brothers into thinking she will never remarry, and then immediately pursues a marriage to Antonio. While high rank and status as a widow render her quite powerful, she does not know that her brothers are conspiring against her and that it will ultimately be them who decide her fate.
