In 2018, Barry Sutton, a detective with the NYPD, witnesses the suicide of Ann Voss Peters, who has FMS—a mysterious ailment in which victims gain alternate memories.

Barry has lunch with his ex-wife, Julia, on what would have been their late daughter Meghan’s 26th birthday.

While investigating Ann’s false memories, Barry is enticed to the strange Hotel Memory, where business magnate Marcus Slade captures him and forcibly sends him back to the day Meghan died in 2007. Barry saves Meghan. He relives his life all the way through 2018.

Once Barry reaches the day he visited Hotel Memory in the previous timeline, the memories from that timeline “catch up,” so Julia, Meghan, and everyone affected by the alternate timeline gain additional memories. Overwhelmed by the memory of dying as a teenager, Meghan commits suicide. A grief-stricken Barry prepares to raid Hotel Memory.

In 2007, Helena Smith, a neuroscientist specializing in memory, begins working for Marcus Slade on an isolated oil rig. She creates a chair that allows the subject to re-experience a previously recorded memory. Slade is not satisfied and pushes the team to invent a deprivation tank that makes it possible to literally reenter memories by dying in the present and traveling back in time. Believing Slade is dangerous, Helena uses the tank to travel back in time and goes into hiding. During this timeline, she meets Barry, and together they plan to stop Slade. Helena realizes that in a previous timeline, she had invented time travel inadvertently, and Slade had killed her to gain control of the technology. Helena returns to 2018 to warn Barry that storming Hotel Memory won’t work.

Together in 2018, Barry and Helena attempt to take over Hotel Memory. During this mission, a SWAT team kills Barry, and DARPA (the Defense Advanced Research Projects Agency) detains Helena. She starts working with DARPA to rebuild the chair, planning to use it for good, but the military takes over the mission. The numerous time shifts lead to mass confusion. Eventually the chair schematics leak, and multiple governments and terror organizations build their own memory chairs. Helena goes back in time to age 16, planning to spend the rest of her life looking for a solution.

Barry wakes up in a new timeline, in which he and Helena have spent their lives unsuccessfully trying to erase the fractured memories. Helena returns to age 16 and tries again. Her next attempt is also unsuccessful, and she repeats the process five more times, under increasing threats of military action of global war. Finally, Barry wakes up in a timeline in which Helena has died, having lost her ability to map memories. Barry is about to accept that they failed, but he realizes he needs to return to the very first timeline and stop Slade from killing Helena.

Barry returns to his original memory of having lunch with Julia. He kills Slade, and then he finds Helena in a bar.