Very dark issues covered here. Everything pieces together - but it's a little convoluted how some of those parts connect - or that's just my naivete talking. 
Reacher is on a train headed to Chicago - but decides to get off when the train stops in the town of Mother's Rest. 
Notes to myself to finish this review after I've had time to digest the ending. 

Chang is looking for her business partner - who actually already ended up in Mother's Rest. Chang and Reacher leave town and go to Keifer's home in Oklahoma City - find a link to the LA Times Science Editor - they figure out who called him about issues - by going through the callers that LA Times guy banned. Leads them to Chicago - to a man who had been murdered just that morning. Talk to his neighbor - retired from UofC and smart. Leads them to man's sister in Arizona. House party - guys come to take them out - Reacher kills them all. Still not clear what is happening in Mother's Rest - what is the hornet's nest that Peter McCann uncovered? LA Times guy connects them to dark web - discover that Mother's Rest has "special sauce" - references to assisted suicide - return to Mother's Rest - people pay for assisted suicide - and then the people in town create snuff films for which they are paid astronomical funds. Still don't get the Russian connection - they help find the buyers of the films??

Very disturbing.