John Thelwall's two plays, written at an early stage of his career, deconstruct cultural myths upholding slavery and empire. An operatic farce, Incle and Yarico comically treats the well-known eighteenth-century story of Inkle and Yarico, in which an English merchant betrays and sells into slavery his Indian lover, represented as a ‘Noble Savage’. The play translates the intertextual narrative into a forthrightly abolitionist satire in its depiction of the slave trade, and wittily represents English middle-class status anxieties and crude materialism, deploying urban middle-class speech and malapropisms. A historical opera, The Incas – a full-length drama designed as a theatrical main piece – translates Spanish Conquest narratives into an anti-imperialistic play, justifying the native rebellions against the Spanish in South America and allegorizing the French Revolution and the English suppression of political dissent. Drawing upon and extending some of the central precepts of the radical Enlightenment, Thelwall undermines the justifications for European empire. The play's hero, known first as Faulkland and then as the historically evocative Sidney, fights with the Incas against the Spanish and becomes embroiled in a complicated but typically operatic love triangle. At the heart of the play is his betrayal and near execution by the Incas for whom he fought, a plot sequence evoking the contemporary revolutionary politics of Paris and London. The play expresses an encounter with the New World mediated by Enlightenment texts that in some sense Thelwall translates. Marmontel's Les Incas (1777) was Thelwall's chief source, but other likely sources include Las Casas (A Short Account of the Destruction of the Indies (1552)), Abbé Raynal (A Philosophical and Political History of the Settlements and Trade of the Europeans in the East and West Indies (1774)), Helen Maria Williams (Peru (1784)), and William Robertson (The History of America (1788)).

The fictionality of Thelwall's New World natives does not mean they are arbitrary, for his knowledge came by way of an intellectual engagement with slavery and empire. His participation in the debating clubs of 1787 during the first wave of the anti-slavery agitation resulted in his losing his Toryism and gaining an abolitionist stance.