Recursion by Blake Crouch, A plot-driven thriller about time, identity, and memory.
In Recursion, Barry Sutton is a detective with the NYPD. New York City has in the past eight months seen a new type of affliction called False Memory Syndrome that leaves its victims with strange memories of other lives.
Meanwhile, Helena is a scientist whose mother has Alzheimer's, and she is working on research involving human memory and mapping the human brain.
As Barry begins to investigate and his path collides with Helena's, he realizes there's a force that's shaping and twisting reality, memories and people's perceptions in ways that are terrifying and horrific in their power.
(The Detailed Plot Summary is also available, below)
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In 2018, Barry Sutton is a detective in the NYPD. It's the day before the 10th anniversary of his daughter Meghan's death. He meets a woman suffering from False Memory Syndrome (FMS), an affliction that causes people to have memories of an alternate reality or former life (what they believe is fake memories). As Barry investigates, he goes to an address to learn the truth about FMS. There, Barry gets strapped into a chair and transported back in time to 2007. He starts reliving everything, except this time he's able to prevent the hit-and-run that killed Meghan, and the remembers the other version of his life as a faded memory.
Meanwhile, back in 2007, Helena is an academic doing Alzheimer's research. She's invited to join Marcus Slade, a wealthy tech founder, who is developing a cure for Alzheimer's. Together, they work on research to re-activate, replicate and enhance memories. By 2009, they've discovered that stopping the heart can make memories more vibrant. Helena watches Slade kill their test subject, Reed. But then it all happens again without Reed's death, while Helena still retains memories of the previous version of events.
Slade explains that they've made a time machine. With it, you can jump to memories where you felt strong emotions. In another reality, he was originally Helena's lab assistant, and they'd accidentally sent someone back in time. When Slade realized this, Slade sent himself back in time to become a tech titan.
Helena and Slade continue to perform experiments on Reed. When Reed tries to kill himself, Slade goes back in time and undoes it. Helena is horrified by Slade's lack of ethics. On July 6, 2009, she sends herself back in time to 2007. This time around, she refuses to join Slade's team and goes into hiding. She knows that (because of how the machine works) on July 6, 2009, Slade will regain his memories of what happened before.
Barry continues living the new version of life, but when people around the world start experiencing large-scale FMS (false memories shared by lots of people), Barry goes to the address he'd been at before to investigate. He is stopped by Helena, who explains that they've met before in a different version of events. Slade ended up rebuilding the device without her after regaining his memories. Barry tried to kill Slade, but the device ended up in the hands of the government.
This time around, Helena wants to help Barry to destroy it completely. They go into the building and meet a version of Slade there who has lived many lifetimes and now his memories are all a mess. The plan goes awry, and Barry and Helena are captured by the government. The government promises to use technology responsibly to prevent small disasters, and Helena reluctantly works with them. Still, the missions get bigger. Then, they start getting false memories of stuff unrelated to their missions, and they know that the technology has somehow gotten out and that other governments and a terrorist organization has it.
Finally, on April 16, 2019, Helena sends herself back to 1986 (to when she was 16) so she can give herself time to figure out a solution to prevent all of it. In the new timeline, Helena and Barry meet, get married and try to figure out how to prevent the old memories from coming back. 33 years pass and when April 16, 2019 rolls around again, everyone in the world regains their memories, so Helena jumps back in time again to try again. She continues doing this until Barry recalls that Slade had mentioned one situation where he didn't get his memories back. The next time around, Helena and Barry abduct Slade.
When April 16 comes around, Slade's memories return to him and he explains that they have to find a way to go back to a dead memory (a memory from a defunct timeline) in the original timeline to prevent all of it, which they'd originally thought was impossible. Helena goes back in time again, but this time around Helena dies from her mind being fractured from all the memories. However, when Barry finally gets his memories back, he manages to reach into a "dead" memory that's strong enough from the original timeline that he can reach it.
In the Epilogue, Barry has returned to November 4, 2018 in the original timeline. Barry goes to Slade's house and shoots him. Barry then goes to find Helena.