Maggie is an 83-year-old woman living alone while her husband is in a memory care facility with Alzheimer's. She's visited by a man who tells her that it's not a disease - her husband's memories are being erased intentionally because of something he discovered, and she needs to go inside his memories to find out what it was and to save him. There's quite a bit packed into this book. It's a thriller, a puzzle book, and especially it's a love story. There's one element of the book that seems more like mysticism than SF, but it's hard to say more - really about any aspect of the book - without spoilers. I enjoyed it and it kept me turning pages into the night.