It centres on Julia Grenville, a Welsh beauty and ingenue (with whom there are parallels with Cavendish herself) who leaves her idyllic rustic life to marry a rich member of the aristocracy.
[1] The letters are chiefly written to her sisters and provide narrative detail about Julia's life in London and her disillusionment with the mores of the inhabitants of the city as well as her miscarriage.
We also discover that she has a long-term admirer, Henry Woodley, that she has growing affections for another man (the Baron Ton-hausen) and also that she has a mysterious and enigmatic protector and guardian, who is the 'sylph' of the title.
[2] The sylph helps provide advice to Julia on the way to negotiate the labyrinth of metropolitan high society, appearing in the work only in the double fictional form of a masquerade.
The book's epigraph is taken from The Rape of the Lock by Alexander Pope and was influenced by The Sylph, a one-act play by Germain-François Poullain de Saint-Foix (1771).