Memory fades with time. Ephemeral humans remember few details of their childhood when they are old. When people live many centuries, nothing of their earlier lives remains. When this story takes place it is two centuries since someone marketed an enhancement to memory that improved capacity and length by a factor of one hundred, allowing those who use it to remember clearly for centuries.
Memory is a powerful thing. One with it can put one without it at a disadvantage. Sudden recall can be staggering in the right time and place. When this story takes place it has been four centuries since a drug was available that allows one to recall the past so vividly, they live in it, and suddenly remember all the details they remembered at the time.
In this story we follow the adventures of Taron, who has enhanced memory but suffers flashbacks as a side effect; and Deneb, who has not enhanced his memory so he can't remember the times they shared. But neither of them remembers the secret they share, a secret that puts them in danger today.