Martin, Elijah Wood, a US student at the University of Oxford, arrives hoping to work under John Hurt as his thesis supervisor. He idolises Arthur Seldom and has studied his work intensely. Martin lodges with Mrs. Eagleton, Anna Massey, an old friend of Seldom, at a house that also hosts her daughter Beth, Julie Cox, a caregiver who resents her role and the attention it leaves her with. Beth’s restlessness hints at deeper tensions, and the house becomes a quiet stage for Martin’s worries about his future and his hero-worship of Seldom.
In a public lecture, Seldom cites Wittgenstein’s Tractatus to argue against the possibility of absolute truth. He leaves the room unshaken, but Martin challenges him, declaring his belief in the absolute truth of mathematics with the provocative line, I believe in the number pi. The clash exposes a chasm between mentor and student, and Seldom’s sharp wit humiliates Martin, who leaves the auditorium feeling deflated and foolish. This moment plants the seed for a dangerous game: the idea that intellectual superiority can be weaponized.
That evening Martin returns to his office and encounters his bitter office-mate, Burn Gorman as Yuri Podorov, who himself failed to become Seldom’s pupil. The mood shifts dramatically when Martin, Beth, and the landlady’s household encounter a murder. The landlady dies, and Seldom insists to the police that a note directing attention toward Oxford is the “first of a series.” As an expert in logical series, Seldom suggests a killer is methodically testing his intellect, even if the crime appears to be an ordinary one. The case becomes entangled with a theory of “imperceptible murders”—victims who were already dying, making foul play easier to miss.
Martin’s personal life deepens in complexity. He visits Lorna, Leonor Watling, who works at a hospital and who shares a complicated past with Seldom. There he also encounters a religious fanatic, Dominique Pinon, whose daughter is in urgent need of a lung transplant. At the hospital, Martin meets Kalman, Alex Cox, a former student of Seldom who has fallen into cancer and madness. The death of a patient sharing a room with Kalman, seemingly by injection, adds to a trail of ominous signs—a second symbol, two interlocking arcs, appearing in the investigation.
As Martin’s relationship with Lorna strains, he begins to see that Lorna once had a romantic history with Seldom. On Guy Fawkes Night, a concert turns tense when Podorov is seen acting oddly and a banner is hung from a rooftop, prompting a police pursuit. The incident ends not with a dramatic arrest but with the death of a musician from respiratory failure, and a drawing of a triangular symbol is found at the scene. Seldom tells Martin a cautionary tale about a 19th-century diarist who plotted to kill his wife, a story that he uses to illustrate that the “perfect crime” is not the one that remains unsolved, but the one solved under the wrong assumption.
Oxford’s mathematical community buzzes with excitement when a local researcher claims to have solved Fermat’s Last Theorem. The scholars—including Seldom and Martin—board a bus to attend a conference, but Martin impulsively leaves as he sees Lorna in the street. The two reconcile and decide to take a long break from mathematics and from Oxford. They share a moment of intimacy, and Martin realizes that the sequence of symbols the killer has sent them points toward a tetractys—the ten-point figure—before the investigation closes in on a shocking revelation.
The police suspect Seldom’s influence, but the true killer is someone connected to the hospital visit: a bus driver who sees children as expendable for his own purposes. He detonates a bomb on a school bus, killing the children and himself, hoping to divert suspicion from his real motive. The aftermath pushes Lorna and Martin toward departure, but Martin discovers that Seldom has not been entirely truthful. Beth, overwhelmed by the burden of caregiving, had killed the landlady, hoping to end the responsibility, and Seldom arrived too late to intervene and instead concocted a story to shield Beth. The hospital death was natural, not injected, and the concert death was an accident that Seldom merely exploited.
Martin confronts Seldom, detailing his discoveries and insisting on the truth he believes he has uncovered. Seldom defends his choices, arguing that even if he lied, his actions harmed no one directly, while Martin points out that Seldom’s lies still have consequences—consequences that ripple outward, influencing others. In a final, morally complex exchange, Seldom contends that all actions carry consequences, intentional or not, and that one casual remark of Martin’s to Beth may have influenced her decision to kill. The story closes on a note of ambiguity, leaving the audience to weigh guilt, mentorship, and the unpredictable costs of truth and power.
> "I believe in the number pi"
