This essay examines the link between Sarah Orne Jewett's "The Mistress of Sydenham Plantation" (1888), a story about former slave-owners and their slaves in post-Reconstruction United States, and a remarkable historical event the story silently invokes through its setting. The enterprise in question is the Port Royal Experiment—a program the Union government launched in 1862 to investigate whether recently freed slaves in the South were capable of being free. When read against this historical context, which inaugurated the capacity for freedom as the criterion of American citizenship, "The Mistress" unfolds into an outstanding interrogation of freedom, personhood, and citizenship—revealing these concepts to be grounded in a restrictive set of values, such as independence, self-ownership, and capacity. By coding its amnesiac former slave-owner and her two former slaves as active participants in the Port Royal experiment, the story registers a discursive shift in post-bellum America from the political and ethical (the right to freedom) to the biological via the economic (the capacity for freedom understood as the ability to participate in the market economy). As a consequence, the story indicates that the notion of "capacity" was inaugurated as the line that separates slaves from persons, and as the criterion not only of African Americans' citizenship but also of their personhood.