Sub-plots:

The sub-plot is dependent on the main plot, although it may not seem so at the time – it might provide twists, shows us how the main character reacts in other settings, provides depth to the story.

They often involve minor characters.

They move the story along toward an apt ending and help the reader get to know the characters better.

Very few novels can survive on one plot alone. Only two come to mind:

My Name is Leon by Kit de Waal - a boy is put into foster care but miss...