The idea for this idiom came to me as a flash when I was reading the story of Saki: A Shot in Dark for eighth graders. Saki is notoriously famous for creating a twist at the end of the story to surprise his readers. In this story: the woman in the story changed the color of her hair from brown to blonde. The story is a lengthy narrative with a man travelling in a train and meeting the son of this lady and another acquaintance. The narrative had little relationship to the plot of the story.
Plot is boiling as an idiom refers to a narrative ploy of the author to create a plot in the end which is very unconvincing to the reader. Potboilers disgustingly adopt this posture.
Example: Jeffery Archer is a villain that sets his stories pot boiling.
The hoi polloi are attracted to pot boiling stories.
For me the aesthetic and philosophical thoughts in fiction are more important than plot is boiling.