Well, said Mary, "I expect he will be back before dinner." But night fell, and Poirot had not returned. 
Mary thought Poirot won't be gone for too long and will be back home soon, but she was wrong. 
mnli hypothesis: Mary thought Poirot won't be gone for too long and will be back home soon, but she was wrong.  premise: Well, said Mary, "I expect he will be back before dinner." But night fell, and Poirot had not returned. 