The play features the same set the entire time – a drawing room in Sidley Park, the home of Lord and Lady Croom (both past and present). Objects remain the same but are added to as the action moves from 1809 to the 1990s.
It is 1809. Septimus is tutoring Thomasina Coverly, the thirteen-year-old daughter. She is a brilliant, precocious young woman studying Fermat’s Last Theorem but is asking him about “carnal embrace” because she has heard rumors of Mrs. Chater, the wife of a local, undistinguished poet and someone in their gazebo. Septimus, ever witty, avoids giving her a straightforward example. Chater himself barges in, challenging Septimus to a duel for being the one in the gazebo with his wife; Septimus calms Chater down by superciliously praising the vain man’s poetry. Noakes, the landscape architect hired to transform the genteel and classically designed Sidley Park into a romantic landscape, comes in with Lady Croom and her brother, Captain Brice. She is critical of the proposed changes.
In the present day, Hannah Jarvis, a scholar, is at Sidley Park researching the famous hermit who lived there in the 1800s. Chloe Coverly, the daughter of the present Lord and Lady Croom, conducts Bernard Nightingale in. He is a don and Byron scholar who once gave Hannah’s book on Caroline Lamb a poor review, so he keeps his identity secret. Chloe’s brother, Valentine, a mathematics graduate student, is also there conducting research on the grouse population. Chloe accidentally reveals who Bernard is, but Hannah still agrees to share information with him because he believes that Lord Byron, who was staying at Sidley Park for a time, shot and killed Ezra Chater. He has found a few letters suggesting a duel, and the only Chater in the historical record after 1809 is a botanist.
In 1809 Septimus is tutoring Thomasina in Latin. They discuss the loss of the library of Alexandria, which Thomasina mourns and Septimus dismisses as irrelevant because all lost knowledge will eventually be found. Chater comes in the room, furious with Septimus because this time he has learned from Lord Byron that Septimus wrote a negative review of his poetry. Frustrated, Septimus agrees.
In the present, Hannah discovers Thomasina’s primer, which trace her early explorations of what would be eventually labeled iteration and chaos theory. Valentine is interested in this but claims Thomasina did not know what she was doing, that she was only doodling. Bernard continues to advance his theory about Byron killing Chater at Sidley Park.
Bernard practices his lecture in front of Hannah, Valentine, and Chloe regarding his theory, which he is preparing for publication. Hannah and Valentine annoy him by challenging him on various points. He and Valentine get into a dispute about science and literature, and Valentine leaves. Hannah starts to wonder if the hermit was Septimus.
In 1809, it is revealed that the duel never happened, and that Lady Croom kicked out Mrs. Chater for sleeping with Lord Byron, and Chater had to go with her. Her brother, Captain Brice, in love with Mrs. Chater, was also gone, as was Byron. Byron leaves the country at this point. Chater will become a botanist for an expedition Captain Brice undertakes, and Mrs. Chater will marry Brice when her husband dies from a monkey bite. Septimus goes hunting for rabbit so Thomasina can have rabbit pie. When he returns Lady Croom chastises him for leaving two letters upon the possible occasion of his death in the duel; one was a love letter to her, and the other was to Thomasina about rice pudding, a metaphor Thomasina used to wonder about time and heat. Lady Croom flirts with Septimus.
The final scene is both past and present, 1812 and the 1990s. In the latter period a ball is being held offstage and the modern characters are all in period dress. Chloe and her brother discuss determinism and sex being the chaotic element in the universe. Valentine is closer to giving Thomasina credit for her theories. Hannah mentions Thomasina died in a fire on the eve of her seventeenth birthday.
Thomasina asks Septimus to teach her to waltz before she turns seventeen. Lady Croom complains about Noakes’s loud steam engine in her transformed garden. This causes Thomasina to reflect on entropy and loss of heat and the universe growing cold.
Evidence is found discrediting Bernard’s theory, which he had just promoted in the media and gained a bit of fame. He is distraught.
Both Septimus and Hannah and Valentine in the future look at Chloe’s drawing of how heat loss cannot be reversed.
Bernard is caught having sex with Chloe and is forced to depart. Chloe is devastated, having fallen in love with Bernard.
Thomasina begs Septimus to teach her to dance, and they do so fluently. They kiss, having clearly fallen in love. Gus, the youngest and completely mute brother to Chloe and Valentine, shows Hannah a drawing Thomasina had made of Septimus with the tortoise Plautus. This confirms the fact that Septimus was the hermit, who went crazy devoting his life to Thomasina’s theories after her tragic death.
The play ends with Thomasina and Septimus dancing gracefully on what will be the night of her death, and Gus and Hannah dancing awkwardly as well.