A writer finds a journal and decides to use its contents for his next novel not knowing it contains hidden plans to murder a future country leader.

It sounds as if the publication is the inciting incident, finding a journal seems almost redundant in the logline.

Perhaps best to change the logline to reflect this:

After accidentally publishing real assassination plans in his novel, a writer must...

As FFF and DPG said, what ever he does thereafter will be the plot, so best to specify this in the logline.

If he doesn't know what the journal is about then there seems to be no plot until he discovers it's true import. ?And then what? ?What is the plot that arises out of this situation? His blithe ignorance or what he does when he realizes the significance of the journal?

Hello,
It seems an 'initial situation': no clues about what the movie is really about.
"When... ? ? then...." What happens?