Complex sentence: Jemima sets about her errand, but the farm collie, Kep, meets her as she carries onions from the farm kitchen and sees through the fox's plan at once.
Split into simpler sentences: Jemima sets about her errand, but the farm collie, Kep, meets her as she carries onions from the farm kitchen. She reveals her errand, and Kep sees through the fox's plan at once.