Joe Rose (the narrator of most of the novel) is picnicking in the Chiltern Hills with his partner Clarissa Mellon when a hot air balloon escapes from its anchor and he joins a crew of onlookers running towards it to help. Joe fills us in on events leading up to the picnic: he has just picked up Clarissa (his partner of seven years) from Heathrow after a six week absence.
They talk of her research project on John Keats as they walk towards the picnic site. Spotting danger the balloon is in, Joe joins two farm workers (Joseph Lacey and Toby Greene), a doctor (John Logan) and a young man (Jed Parry) in an attempt to help the balloon's pilot (James Gadd) bring under control the balloon containing his grandson (Harry).
As they are each holding a rope, a gust of wind lifts the balloon into the air, and all but Logan drop to the ground. Logan holds on as the balloon rises, then his grip fails him and he falls to his death.
The reaction to the accident. In shock and after some preliminary discussion with the others and Clarissa (who encourages him to slow down), Joe sets off towards the point where Logan fell. Joe describes the body.
Jed Parry eventually follows him (but not at Joe's initial prompting), and after some awkward and presumptuous conversation, invites Joe to pray with him. Joe refuses repeatedly. The police arrive.
Back in their London flat, Joe and Clarissa repeatedly relive the event in narratives, constantly moving towards and then drawing away from Logan's death. There is some back-story on their childlessness - the result (at least in part) of a bungled operation on Clarissa due to mistaken identity.
They make love, then tell each other stories of there childhoods. They then invite friends around and tell then the story of the day. They go to bed at 2am, and five minutes later the phone rings: it is Jed Parry telling Joe that he loves him. Joe tells Clarissa it was a wrong number.
The next day. Clarissa leaves for her job at a university, whilst Joe- who appears to work from home - writes an article on the Hubble telescope. He has an interview with a radio producer in Soho about a potential radio programme on supermarket vegetables: Joe tells the producer about the events of the day before, but he is not interested.
Joe goes to the London library to read some of Darwin's contemporaries, masters of narrative science, and feels a sense of apprehension. Someone leaves and he goes outside to see who it was, clearly thinking Jed Parry has been following him. He replaces some knocked-over flowers marking the spot WPC Yvonne Fletcher was shot outside the former Libyan Embassy, then returns inside.
Later that day. After a second meeting of the day about judging a science book prize, Joe returns to the flat desperate to talk to Clarissa about Jed. She is not at home, and he works to relive his anxiety, writing a piece on how the narrative science of the 19th Century was replaced by the theoretical science of the 20th Century, but conceding that it is possible to write a very different and contradictory narrative of this development.
Clarissa returns and tells him about her brother's infidelity and impending divorce: he doesn't tell her about Parry, and has unplugged the phone to try to keep him away.
The next morning. A description of Joe and Clarissa's appointment building, where Joe is having breakfast on the roof garden and thinking about the accident. Clarissa joins him and he tells her about Jed's phone call and the London library incident: she laughs it off.
Clarissa leaves and Jed phones instantly, asking Joe to meet him. Joe agrees, and is comforted by the smell of Clarissa's perfume as he leaves.
The second meeting. Joe meets Jed on the pavements outside his flat. Jed tells Joe that his mission is to bring him back to God, but claims that Joe has instigated this contact and is manipulating and controlling him.
Joe feels himself being drawn into a relationship with Jed despite himself - though not the relationship Jed is after. Joe finally loses his patience, hails a taxi and leave abruptly when Jed mentions dealing with what he sees as the 'Clarissa problem'.
In the taxi into town to pick up a book, Joe reflects on what has happened, and then, based on an article he is due to write on the smile, discusses how the standard social science model of the mid-twentieth century is being superseded by the more hard-core sciences of evolutionary psychology, neo-Darwinism and genetics - a development Clarissa rejects as 'the new fundamentalism' and 'rationalism gone berserk'. Joe returns to the flat where Jed is waiting for him, and where he phones him repeatedly throughout the day.
Joe phones the police but they mock his concern. Joe fills us in on his past - failed theoretical physicist turned science journalist, regretting the parasitical nature of his trade, and wishing to get back to his old life. In Jed's 29 phone calls, there is one about Joe signalling to him with the curtains.
Clarissa's return home (narrated in the 3rd person and from her point of view). She's had a bad day, feels ill, and just wants a bath and for Joe to look after her; but he tells her all about Jed and his desire to get back into theoretical physics, giving her no space.
She doesn't believe him about the former, going as far as to suggest Joe might have invented him, and is doubtful about his plans. Joe storms out, only to meet Jed at the end off the path.
Jed follows Joe and tries to talk to him, as Joe imagines he is someone else, and tries to recall the significance of the curtain reference.
Joe loses Jed.
Jed fills in Joe on what he calls his 'ocean surface' - from ordinary family but living in a palatial house in Hampstead due to a crook of an uncle - having already revealed to him his 'ocean floor' - his soul.
He reiterates his mission (to 'mend [Joe's] rift with God through the healing power of love'), mentions again the problem of Clarissa, apologises for what he sees as rejecting Joe on their first encounter after the ballooning accident, and asks for Joe's forgiveness.
Joe is driving to Oxford to see John Logan's widow, Jean. In a flashback he recounts his breakfast conversation with Clarissa about Jed's letter, a conversation in which it appears they have very different views about the situation and are drifting apart because of it. Joe begins to suspect that she has a lover and, after a long discussion of 'unacknowledged self-persuasion', finds himself in her study and going through her private letters.
After a brief flash return to the journey narrative, there is a brief flash forward to Clarissa's return home that night and the arrival of a letter the following morning turning down his bid to re-enter the world of theoretical physics. Back in the journey narrative, he approaches Jean Logan's house and begins to realise his true motives for the visit.
On arrival Joe reads the house as 'a perfect setting for sorrow', but once inside sees that it has other, older stories to tell too.
Jean Logan quizzes him about the accident, and clearly suspects her husband was having an affair and was with the woman at the time. She needs certainty, despite knowing it will be painful, and wants revenge.
Joe engages in a discussion of moral relativism with the Logan children, then Jean asks him to contact the others present at the ballooning accident to see if they have any information. She also tells him her theory of why John Logan died - not, as Joe fears, because he or one of the others let go of the ropes too soon; but because Logan was showing off to his new woman, and trying to deny the aging process.
This conversation takes place over the noise of the children playing with the curtains, which triggers Joe remembering about De Clerambault's syndrome - a piece of knowledge to cling onto which makes him 'almost happy'.
On the way back from Oxford, Joe revisits the scene of the ballooning accident, imagining De Clerambault is there. Jed is waiting for him outside the flat and tells him that he spent the previous evening reading Joe's 35 articles.
Joe also feels slightly threatened by Jed's claim that he can pay to get what he wants. Inside, Clarissa is waiting in his study to confront Joe about his going through her private letters.
Jed criticises Joe's work as an attack on God, but is more determined and sure of what he sees as his ultimate victory. He reassures Joe that he will always be there for him, and warns him against trying to pretend that he (Jed) doesn't exist.
Joe is in bed with Clarissa, thinking he is enjoying a recovery of their disappearing intimacy when she tells him that it's all over and departs for the spare ('children's') room.
This narrative is interspersed with details about Jed's other (frequent) letters and loitering, reflections on Jed's character and 'inviolable… solipsism', and details of Joe's phone calls to the others present at the accident made at Mrs Logan's request. Most draw a blank, but Joseph Lacey clearly knows something and agrees to meet Joe.
The day of Clarissa's birthday. Joe prepares a file for the police of what he sees as Jed's threats, but his interview with Duty Inspector Linley gets him nowhere. Joe, remembering the morning of Clarissa's birthday, muses on religion and genetics, and evolution.
Clarissa's birthday lunch with Joe and her godfather, Professor Jocelyn Kale. Kale gives Clarissa her present - a double helix brooch - and regales them with tales of the discovery of DNA.
Joe finally gives her his present - a first edition of Keats' Poems of 1817. A man in the party of three on the next table is shot and wounded, and a lone man (Jed Parry) at another table prevents the would be assassin from finishing off his victim.
Back at the police station, and grateful and relived that at last the truth will out, Joe is shocked to discover that his version of events is not believed by the police - not helped by the fact that everyone's version is slightly different.
Joe reflects on the impossibility of objective truth. Back at the flat Joe searches through his address book, finally finding the person - Johnny B Well - most likely to be able to get him what he wants - a gun.
Joe and Johnny go to pick up the gun from Johnny's ex-hippy contacts, Steve, Xan and Daisy.
After a tense and violent encounter Joe leaves with the gun, only to be phoned by Jed who is in Joe's flat with Clarissa.
Joe stops in some woods to try out the gun, races back to London, enters the flat and, after some tense moments with Jed and a knife, shoots Jed in the elbow.
No charges are pressed against Joe, but he lives with an enduring image of Clarissa's expression of 'repulsion and surprise' at his using a gun.
Clarissa's letter. Whilst acknowledging that he was in some ways right about Jed Parry, and that he has saved her life, Clarissa nevertheless still blames Joe for the way he has handled the situation, and fundamentally sees it as his fault. From her point of view Joe, as well as Jed, has serious problems.
Joe and Clarissa drive to Oxford to see Jean Logan, ten days after the shooting and the day after Joe's meeting with Joseph Lacey. They talk awkwardly and impersonally about Keats, electric cars and life on Mars before arriving at Mrs Logan's.
After a picnic by the Thames they are joined by James Reid, Euler Professor of Logic, and Bonny Deedes, his student. Reid explains that Bonny is his 'mistress', and was not Logan's, who was merely giving them a lift after their car had broken down.
The main narrative ends with Jean Logan's daughter Rachael asking Joe to tell her brother Leo about the river.