The plot concerns two – literally starcrossed – lovers: Swithin St. Cleeve, a very young amateur astronomer, and Viviette Constantine, an unhappily married and abandoned woman 8 or 9 years his senior. Each night Swithin climbs the old tower of the title, in the grounds of the Constantine estate. Lady Constantine, whose husband has been absent some years on an extended hunting and exploring journey to Africa, joins the young man in his stargazing, and supports his astronomical ambitions by buying him equipment, though his dreams of scientific renown are disappointed.Their relationship then deepens and takes several twists and turns.