Ptolemy Grey is a 91-year-old black man living alone in a disheveled apartment in L.A. He's suffering from severe dementia, but he's at just the stage where he remains conscious of how he's losing his memories. When he talks to people, he's aware that they don't understand him. Nevertheless he's stuck in this apartment and dependent upon the kindness and aid of people who can barely understand his needs much less his inner state. His great-grandnephew, Reggie, cares for him until one day Reggie stops coming. He died unexpectedly.
As Ptolemy struggles to wade through his mental fog, he notices that Reggie is gone. His neighbor starts stopping in to check on him, but she robs Ptolemy. He knows this, but he can't do anything about it. Eventually, Robyn, his great-grandniece, a teenager, stops in to take over for Reggie. She brings light back into Ptolemy's world, first literally by opening the windows and cleaning up, and the figuratively by reaching into his mind and showing him kindness. As his situation improves, Ptolemy's memories also start returning piece by piece. He's triggered by various items in the apartment or an outing on the bus.
Eventually Ptolemy confides in Robyn that he is the guardian of a treasure, with which he was entrusted something like 80 years ago, but he doesn't remember why or where. He remembers that this was really important. Around the same time, he begins to seriously question Reggie's death. He learns that perhaps it was not accidental and becomes determined to find justice for his old companion, while simultaneously relying increasingly more upon Robyn. The two enjoy a very intimate relationship. She takes him to a doctor one day, and together they learn that there is an experimental drug which will help his dementia but it will also reduce his life expectancy to a few months. He gives an enthusiastic yes. With the newfound clarity which the drug provides and with Robyn's devoted assistance, Ptolemy does solve both mysteries before his sudden and painful death.
