The play is set in "the province of Andalusia in Spain". The opening scene introduces Duke Angelo and his elder son and heir, Roderick. Roderick is the dutiful and virtuous son; the Duke also has a younger son, Henriquez, a scapegrace and prodigal who is absent from the ducal court, pursuing his own interests. Henriquez has just written his father a letter, requesting gold to buy a horse; Henriquez will send his friend Julio to court to receive payment. The Duke and Roderick decide to use Julio for their own purposes: they will detain him at court "some few days...and assay to mould him / An honest spy" upon Henriquez's "riots".