When sixty-something Leo Colston rummages through his old school belongings, he finds his younger self’s diary from fifty years earlier—when a twelve-year-old Leo spent a fateful summer at the Brandham Hall estate in Norfolk. Feeling the advance of his age, Leo decides that now is the time to confront the memory of those events that he has repressed for so many years—events that have ruined any chance he once had at an “emotional” life.
Cast back to the year 1900, and young Leo is a naïve, curious schoolboy. He’s obsessed by the Zodiac and, owing to a recent success in “vanquishing” bullies at his school with a magic spell, believes he has supernatural powers. One of his school-friends, Marcus, who has a newfound respect for Leo on account of his magical reputation, invites Leo to stay at his aristocratic estate in Norfolk for the summer. Leo accepts, though is apprehensive—he leads a humble home life and isn’t accustomed to the refined world that the Maudsleys—Marcus’ family—and Brandham Hall represent. His mother, who raises Leo single-handedly, feels she will miss Leo but insists that he must go.
Leo arrives in the opulent environment of Brandham Hall in July and is quickly intoxicated by its inhabitants and their refined way of life. He wanders freely around the grounds, and in one disused outhouse discovers an enormous Deadly Nightshade—a highly-poisonous plant once used in witches’ brews. Marcus shows Leo a thermometer in another of the abandoned buildings, starting Leo’s almost daily obsession with checking the temperature.
Leo takes a particular liking to Marcus’ beautiful older sister, Marian, who offers to take him to nearby Norwich and buy him clothes better suited to the intensely hot weather. On their day-trip, Leo is quickly smitten with Marian, who buys him a green suit that makes him feel a bit like Robin Hood (with his Maid Marian!). While in Norwich, Marian asks Leo to entertain himself for a while by visiting the cathedral; just before they reunite to head home, Leo notices her saying goodbye a male stranger.
Though he isn’t allowed to swim yet, Leo joins some of the inhabitants of the Hall who decide to go bathing in a nearby sluice. As the group arrives, they notice the imposing physical presence of Ted Burgess, a nearby tenant farmer on the estate, already in the water. Denys, Marcus’ dim older brother, has a brief conversation with Ted, informing him that Lord Trimingham, the estate’s landowner, will arrive later that evening. Leo is fascinated by Ted’s impressive physicality, but Marian pays him no attention.
Leo meets Trimingham at breakfast on Sunday morning. Trimingham has recently returned from fighting in the Boer War in which he sustained an unsightly injury to half of his face. After breakfast, Leo goes back to his room to fetch his prayer-book for church. There, Marcus informs him that he has the symptoms of measles and won’t be attending the service and may even miss the upcoming cricket match and ball (rare occasions when the people from the Hall mix with the lower-class villagers).
Leo heads to church with the other guests and afterward walks home with Trimingham. Leo realizes that Trimingham is a Viscount and has a newfound respect for him; Trimingham in turn asks him to let Marian know that she has left her prayer-book in church, setting up Leo’s role as a messenger. On a later day, Trimingham christens Leo with the nickname “Mercury,” who is the messenger of the gods. Leo loves the idea, mapping Ted, Trimingham and Marian on to the Zodiac as the Water-carrier, the Archer and the Virgin, respectively.
With Marcus now quarantined, Leo is given his own room. Feeling newly independent, he explores the estate and ends up on Ted Burgess’ farm. He slides down a huge pile of straw, but injures his knee in doing so. Ted, upon learning that Leo is from the Hall takes him into the farmhouse to tend to his wound. As a gesture of thanks, Leo agrees to take a note from Ted to Marian, which Ted insists must be done secretively. Marian is excited to receive Ted’s message and reiterates that if Leo tells anybody about it they would all get into a lot of trouble.
The weather continues to grow hotter, pleasing Leo. One day, Trimingham tasks Leo with finding Marian so she can join in a croquet game. When Leo finds her she’s reluctant to play—but she does give Leo a letter pertaining to “business matters” for him to take to Ted. Over the next few days, Leo continues to carry notes and verbal messages between Marian and Ted, unwittingly facilitating their secret relationship. Marcus begins to recover, and Leo thinks he won’t be able to take any more messages without arousing Marcus’ suspicions. Marian gives Leo another letter but is hurried by Trimingham’s entry into the room. As the envelope is unsealed, Leo looks inside on route to the farm—he’s horrified that it’s a love letter.
Leo delivers the letter to Ted but says he won’t be able to take any more. Ted insists that Leo keep taking them; otherwise Marian will be distraught. Leo asks Ted about “spooning”—sex—and Ted promises to tell him more if Leo keeps acting as the lovers’ go-between.
The day of the cricket match arrives and Leo, drafted into the Hall team on account of another player’s injury, catches out Ted, who is the star batsman of the village team. In the village hall afterwards, both Ted and Leo sing songs accompanied by Marian on the piano. Leo’s is a star turn, and he basks in the glory of his achievements in both the cricket match and the party afterwards. On the walk home, Marcus lets Leo in on the news that Marian is now engaged to Trimingham, which Leo thinks will surely bring an end to Ted and Marian’s letters.
After the next Sunday church service, Leo asks Trimingham about the Viscounts that came before him. Leo learns that the fifth was killed in a duel over a woman, but Trimingham warns him that “nothing is ever a lady’s fault.”
The next day, Leo is shocked when Marian gives him another letter for Ted. Feeling loyalty toward Trimingham, he tells Marian that he can’t take it for her, to which she reacts with anger and accuses him of being spoilt. In tears, Leo grabs the letter and runs to the farm. When Leo arrives, Ted is staring down the barrel of his gun, cleaning its insides. Ted notices that Leo’s been crying and assumes it is to do with Marian. Ted manages to cheer Leo up, but gets frustrated when Leo questions him relentlessly on love, marriage, and “spooning.” Ted stands up intimidatingly, causing Leo to run away back to the Hall.
Leo writes to his mother asking to be called home, without going into too much detail why. He thinks that if he can remove himself from Brandham Hall it will bring an end to Ted and Marian’s affair. Trimingham asks Leo to find Marian for him, but she is at the house of her grandmother, Nannie Robson—at least, that it is what Leo heard from Marcus. Trimingham accidentally lets slip that Mrs. Maudsley is ill.
Leo spends some time with Marcus, who tells Leo that his mother is under nervous strain because she feels that Marian may go back on her agreement to marry Trimingham. He also informs Leo that for Leo’s upcoming birthday Marian has bought him a green bicycle. Through Marcus’ teasing, Leo realizes that the color green implies that he is young and naïve. Annoyed, Leo boasts that he knows where Marian actually is. They head to one of the outhouses and hear two people in there, but Leo prevents their discovery by saying he’s bored.
Marian goes to London, and Leo enjoys a couple of carefree days. Invited into the secretive world of the men’s smoking room, Leo learns from Trimingham that Ted might join the army and go to fight in the Boer War. Thinking that he will soon be leaving, Leo goes to visit Ted to say goodbye. As a parting gesture, Leo offers to take one last message for Ted. On returning to the hall, Leo receives a letter from telling him to stick it out with his stay at Brandham Hall—it would be rude to leave early.
When Leo next sees Marian, he changes one crucial detail of Ted’s message. He tells Marian that Ted wants to meet on Friday (the day of Leo’s birthday party) at 6:00 p.m., when in fact Ted had said 6:30. He mentions that Ted will be going to war, which greatly distresses Marian. Leo asks why she can’t just marry Ted—she say’s it’s impossible. Leo sneaks out late one night and pulls up the Deadly Nightshade, wanting to use some of it in a spell to break Ted and Marian apart.
The day of Leo’s birthday arrives; for once, the weather is overcast. Marian takes Leo aside to give him a letter but is interrupted by a suspicious Mrs. Maudsley. Marian insists the letter is for Nannie Robson to inform her that she will visit her in the afternoon. In the late afternoon, the guests gather to celebrate the birthday, but Marian is nowhere to be seen. When a carriage for Marian returns from Nannie Robson’s only with the message that she hasn’t been there at all, Mrs. Maudsley drags Leo out to look for her. In the pouring rain, they arrive at the outhouse where the Deadly Nightshade had been. They discover the entwined bodies of Ted and Marian, making Mrs. Maudsley scream. Subsequently, Ted commits suicide with his gun.
The narrative returns to old Leo, who heads back to Norfolk to find out what happened to those involved. Leo meets Marian’s grandson—the eleventh Viscount—who arranges for him to meet Marian. He learns from her that Marcus died in the first world war; Trimingham married Marian despite her affair and treated her love child with Ted as his own but died ten years later. Mrs. Maudsley, like Leo, never really recovered from the trauma. Marian tells Leo that her grandson believes he has cursed because of what happened at Brandham Hall and begs Leo to tell him that her and Ted’s love was pure and beautiful. He reluctantly accepts this final mission as go-between and heads towards Brandham Hall, feeling himself to be “a foreigner in the world of emotions.”