When Christopher returns from questioning his neighbors, he only tells his father that he was “out,” which he thinks of as a white lie, because he’s not telling the entire truth. However, Mrs. Shears has already called Ed and told him what Christopher was doing.
This incident marks Christopher’s first departure from strict truth-telling. He rationalizes his statement as a partial truth, but is nonetheless stretching his own rules after repeatedly insisting that he always tells the truth. His detective work is changing him.
Ed is angry with Christopher for continuing to pursue the mystery of Wellington’s death. Christopher tells his father his suspicion that Mr. Shears killed the dog, and Ed gets even angrier with Christopher for mentioning Mr. Shears. He also says that Mrs. Shears is no longer their friend. Finally, he makes Christopher promise to stop looking for Wellington’s killer. Christopher takes promises very seriously, and he agrees.
Although Christopher does not wonder at his father’s anger towards Mr. and Mrs. Shears, the reader can perceive that something more is going on between Ed and his neighbors than Christopher knows. Yet at this moment that verges on revelation, Christopher’s detective work seems doomed, as his dedication to truth and trust make a break with his promise seem impossible.