The following version of this book was used to create this study guide: Mandel, Emily St. John. Sea of Tranquility. New York: Alfred A. Knopf, 2022.

In Part 1 of Sea of Tranquility, in 1912, an 18-year-old man named Edwin St. John St. Andrew arrives in Canada, where he has been exiled by his wealthy British family after making disapproving comments about the colonization of India over dinner. Edwin is uncertain what to do with his life, and he ends up on a small island called Caiette, where his friend is interested in a logging operation. Walking through the forest one day, Edwin stops near a large maple tree and sees a man dressed as a priest, who introduces himself as Roberts. Suddenly, Edwin hears violin music overlapping with the sound of a train and has a strange sensation of being in two places at once. He flees the forest and goes to the church, where he is asked about this experience by Roberts. Edwin is suspicious of Roberts' story that he is filling in for the church's usual priest, and when he asks questions, Roberts flees.

In Part 2, in 2020, a woman named Mirella Kessler attends a musical performance by a man named Paul Smith. Paul is the brother of a former friend of Mirella's, Vincent Smith. Vincent's husband, Jonathan, launched a phony investment scheme that cost Mirella's husband, Faisal, his life savings. Faisal later died by suicide. Mirella has begun to wonder if Vincent perhaps knew nothing of Jonathan's scheme, so she is seeking Vincent out through her brother. However, after the concert, Paul tells Mirella that Vincent has been dead for several years. Another man who came to the performance, Gaspery-Jacques Roberts, asked Paul about the video he had shown. He explained that it was shot by his sister in the forest of Caiette, where they had grown up. Mirella suddenly remembers that she has seen Gaspery before — in an Ohio underpass when she was a child, holding a gun while two men bled to death nearby.

In Part 3, in 2203, a woman named Olive Llewellyn, resident of Moon Colony Two, is on a book tour on Earth. She is promoting her book, Marienbad, a novel about a pandemic, and a real-life pandemic has just broken out on Earth at the same time. She continues the tour, despite her concerns, speaking to her husband, Dion, on the phone periodically. Dion is an architect, and he tells Olive he is working on a university building that is connected to a police station by underground tunnels. Olive posits that the building might have something to do with time travel. Later, she is interviewed by a journalist named Gaspery-Jacques Roberts, a unique name that she was certain she had invented for one of the characters in Marienbad. Gaspery asks about a scene in the novel that features a character in a spaceport who hears a violin and feels as though he is in two places at once. He asks if something strange happened to her in the Oklahoma City Airship Terminal and Olive asks if they can go off the record.

In Part 4, in 2401, Gaspery grew up on Moon Colony Two but moved to Moon Colony One as an adult. He gets a job as a hotel detective, hired by a woman named Talia he had known as a child. Gaspery visits his sister, Zoey, at work on her birthday. She works at the Time Institute, a government facility devoted to time travel. Zoey tells Gaspery about Edwin, Vincent, and Olive, all of whom live in different times and places and all of whom have experienced the sensation of being in two places at once while hearing violin music and hydraulics. The Institute is calling this phenomenon the anomaly and considering launching an investigation, which will require a time traveler to visit each person and interview them. Gaspery asks to be hired for this role and Zoey says that it is too dangerous. She also tells him that the anomaly may be evidence that humankind is living in a simulation. Another day, Gaspery returns to the Time Institute and runs into an old friend named Ephrem. He asks Ephrem about the job and Ephrem agrees to hire him. Zoey explains that time travelers must never act in a way that affects the timeline, even to save a person's life, and says that the Institute has been known to strand people in other times for breaking the rules. She notes that Olive is going to die in the pandemic just days after he interviews her. Talia tells Gaspery that her parents were time travelers and that, “when the Institute is done with you, they'll throw you away” (151). Gaspery trains for five years and begins his first assignment: interviewing a man named Alan Sami who plays violin in the Oklahoma City Airship Terminal in the 22nd century. Alan tells Gaspery that he has enjoyed playing in the terminal since his wife died several years earlier.

In Part 5, Gaspery implies to Olive that he is a time traveler and says that she must return to Moon Colony Two immediately. Shortly after she returns home, the pandemic arrives in Colony Two and Olive, Dion, and their daughter Sylvie are on lockdown. Olive is haunted by the realization that she was meant to die in the pandemic.

In Part 6, Zoey tells Gaspery that he has attracted the Institute's ire by saving Olive's life and altering the timeline. Gaspery goes to Caiette in 1994, where he watches a teenage Vincent Smith film the video of the anomaly. He then visits an art gallery in 2007 and speaks to an adult Vincent about her experience with the anomaly.

In Part 7, Edwin returns to England in 1918 after fighting in World War I. His brothers, friends, and lover have all died in combat. Gaspery visits Edwin and tells him that he did not imagine the anomaly in Caiette. Gaspery then returns to the Time Institute, where he is drugged by Ephrem and then taken to an underpass in Ohio in 1990. He realizes he has a gun in his hand and two men lay bleeding nearby. Mirella, as a child, walks into the underpass. Gaspery is imprisoned for murder.

In Part 8, well into his prison sentence, Gaspery is visited by Zoey, who tells him that she has left the Time Institute and is now working for a different time travel organization. She takes him to Oklahoma in 2172 and gives him a new identity. He has facial reconstructive surgery and realizes he looks exactly like Alan Sami. He discovers that Talia is living on a nearby farm and the two marry. Gaspery realizes that when he interviewed Alan in the Oklahoma City Airship Terminal, he was interviewing another version of himself, and that this is what caused the anomaly. He also realizes that Talia will die before him, since Alan told him he was a widower during the interview. Talia dies at 75 and Gaspery moves to Oklahoma City, where he begins playing violin in the Airship Terminal. He has the interview with his younger self, and thinks of Olive, Vincent, and Edwin.