Lose the drift/thread of something - Idioms by The Free Dictionary
lose the drift/thread of something
lose the drift (of something)
To stop being able to understand or follow something, such as an explanation, because one has become distracted or confused. Sorry, can you back up? I lost the drift of the topic when you started talking about genetic formations. I think this writer is trying to intentionally make us lose the drift so that the narrator becomes untrustworthy.
lose the ˈdrift/ˈthread of something be unable to follow a story, discussion, etc. because you cannot understand the relationship between events, facts, etc: I had to go out in the middle of the film and when I came back I found I’d lost the thread entirely. ♢ When they started talking about artificial intelligence, I completely lost the drift of the argument.