Cassie is travelling with her boyfriend from Bangkok to Afghanistan in 1978. En route she exchanges letters with a Phd student in the States who is a few years older than she is. In Kabul they meet an American and a Canadian, both young men who also plan to go to Iran. (Then called Persia and not speaking Arabic - the only slips I noticed).
Thirty years later the pen pal is a professor and rediscovers those letter while packing his office to move buildings. He had till then always assumed she ended her friendship with him but on the brink of a divorce he finds out someone had done that for him. Curious he googles her and finds out she vanished. (And we are still reading her adventures in 1978 and are none the wiser but are looking out for clues).
The book is an interesting roadstory - I even told my Afghan refugee students the words in Dari I learned from it. The other storyline is about a middle aged scientist who turns detective.
Can really recommend it.