lose the thread
Cease to follow the sense of what is said. For example, It was such a long story that I soon lost the thread. This expression uses thread in the sense of “something that connects the various points of a narrative.” [Mid-1900s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.