The Cutting Room — Edward W. Robertson

In the future, there are many parallel Earths -- but only one of them has time travel. Known as Primetime, its criminals break into the pasts of other worlds, far beyond the reach of conventional police. Blake Din is the top agent of the Cutting Room, the group tasked with stopping these trespassers. After a mission to undo the murder of a young boy, he uncovers extensive damage to another world's timeline -- and a conspiracy that reaches all the way back to Primetime.