On assignment in Thailand, French television journalist Marie Lelay (Cécile de France) is shopping for souvenirs for her lover's children. She finds a stand where a mother and her daughter work; they sell gifts to Marie for a dollar. Her lover Didier (Thierry Neuvic) looks over the balcony and witnesses the 2004 Indian Ocean tsunami coming into shore. It hits as Marie watches from a distance. She grabs hold of the girl and runs away from the shore but is quickly swallowed by the wave.
Pulled lifeless from the water, she is resuscitated by rescuers but is left for dead. She gasps back to life after having a near-death experience in which she sees a vision of human figures inhabiting a realm of light, among them the silhouettes of the mother and daughter holding hands. Marie and Didier are soon reunited as the disaster subsides and they return to Paris. Marie's experience, however, interferes with her work performance to the point that Didier (who is also her producer) sends her on a leave of absence to write the book they've discussed, which would add to her prestige.
The story then turns to San Francisco where former professional psychic George Lonegan (Matt Damon) is persuaded against his wishes to perform a reading for Christos (Richard Kind), a wealthy client of his brother Billy (Jay Mohr). A genuine medium with a gift for communicating with the dead, George abandoned his old career because he was unable to deal with the emotional impact of the reunions and the often disturbingly intimate family secrets revealed.
While doing the reading, George hears the word June and asks if a date in June means anything to him. Christos at first denies that it means anything, but privately reveals to Billy that June was the name of his late wife's nurse, whom he was in love with for ten years.
From there, 12-year-old London twins Marcus and Jason (Frankie and George McLaren) try desperately to prevent their alcoholic, heroin-addicted mother, Jackie (Lyndsey Marshal), from losing them to social services. After evading the authorities yet again, the boys' mother sends Jason to the chemist (pharmacist) to pick up her detox prescription. On the way home, Jason is attacked by street thugs, and while trying to escape, he is hit by a van and killed. No longer able to protect his mother, and barely able to cope with life without the brother he idolizes, Marcus is sent to a foster home.
Now writing a book and with more time to contemplate her near-death experience, Marie travels to Switzerland to meet a renowned specialist in the field.
As the director of a hospice who has seen her share of dying patients, the doctor describes herself as a former skeptic who was convinced by the evidence that the afterlife exists and that people like Marie have had a genuine view of it. She persuades Marie to write a book on her experience in the hope that the scientific community will ultimately accept the reality of life beyond death.
Desperate for one last reunion with his twin brother, Marcus steals money from his foster parents (Niamh Cusack and George Costigan) and goes around London seeking someone to help him contact Jason. He encounters only frauds and pretenders. While he is trying to board the underground at Charing Cross, Jason's cap, which has become a talisman for Marcus, blows off his head. Delayed by trying to find the cap, he misses his train and sees it explode in the tunnel during the 2005 London Bombings.
George enrolls in a cooking class taught by one of San Francisco's leading chefs. Its students are paired-up, resulting in George being partnered with a young woman named Melanie (Bryce Dallas Howard). The two soon hit it off and after attending their second class decide to put their new culinary skills to use by preparing an Italian dinner at George's place.
All goes well until they hear an ill-timed phone message from his brother, which inclines George to reveal his past as a psychic to Melanie. Curious, she presses George to do a reading for her. George explains his reluctance, but acquiesces. They contact the spirit of Melanie's father, who ends the session by asking her forgiveness for what he did to her as a child. Melanie flees George's home in tears, and she doesn't return to the cooking class.
Having been in talks with a publisher before her trip to Thailand about a biography of François Mitterrand, Marie now stuns them with her new manuscript entitled "Hereafter: A Conspiracy of Silence". The publisher (Jean-Yves Berteloot) rejects the manuscript but soon steers her toward other publishers who might be interested, the most promising of them in London.
Marie learns from Didier that he does not intend on having her back at the job he urged her to take leave of, because her public interest in the hereafter damages her reputation as a serious journalist, and that he is having an affair with the woman who replaced her on the TV news program.
George is laid off from his factory job, and is persuaded by Billy to revive his psychic practice. Still heartbroken over the fiasco with Melanie, he changes his mind and impulsively leaves San Francisco to make a new start elsewhere. He travels to London and listens every night to audiobook readings by Derek Jacobi of Charles Dickens' works.
As a Dickens devotee, he also visits the Dickens Museum and attends a live reading of Dickens at the London Book Fair. There, one of the presenters is Marie, reading her now published book, Hereafter. While handing a signed copy of her book to George, their hands touch and George has a psychic flash of Marie's tsunami drowning.
Marcus and his foster parents are also at the London Book Fair. Asking leave of them, Marcus spots George, someone he has read about and seen online. Marcus attempts to speak with the medium, who brushes him off and returns to his hotel. Marcus follows him, standing outside the hotel until nightfall. Eventually George asks him in and agrees to do his reading.
Through George, Jason tells Marcus that he is happy in the afterlife. He instructs Marcus to stop wearing his cap and says it was he who knocked it off his head at the train station. It was used, he says, to keep Marcus from the doomed train but now he must stand on his own. Jason tells him not to fear this "because we are one".
As Marcus leaves George, he says he is sorry about "the French woman" as he could tell that "you like her." The last time we see Marcus, he is visiting his mother in a rehab center. She is visibly better and he is not wearing Jason's cap.
Marcus lets George know where Marie is staying. George leaves an anonymous note for Marie, saying he believes her book to be true. She decides to join the anonymous fan for lunch and discovers George. While she is looking for him, George sees a vision of them kissing at the same meeting. Their shared glimpses of the hereafter having made them appreciate this life all the more, George and Marie walk away hand-in-hand.