The specialist, when they finally found one willing to take their unusual case seriously, turned out to be connected, in a coincidence neither of them fully trusted at first, to Marcus Cole’s foundation — a soft-spoken geneticist named Dr. Amara Osei who had spent the last several years researching precisely the kind of hereditary memory conditions Cole’s own grief had originally set the foundation in motion to address.

Her office was smaller and less clinical than either of them had expected, warm with personal photographs and the particular clutter of someone who spent more time thinking than tidying, and she greeted them both with a directness that put Lucien considerably...