Back in the present, Christopher gets home from his investigative word and his father is sitting at the kitchen table.
Christopher has to tell a "white lie": he says he's been "out" (79.3).
Unfortunately, his cover is blown: Mrs. Shears just called and told him that Christopher was snooping around in her yard again.
His dad pretty angry, since he specifically told Christopher "to keep [his] nose out of other people's business" (79.10) – obviously this doesn't include poking around someone else's property.
Christopher tries to reason with him – after all, Mr. Shears is his Prime Suspect. This doesn't go over very well with Dad: he totally flips out, banging the table and yelling that he never wants to hear Mr. Shears' name mentioned in his house. That guy is evil.
Christopher says he knows he shouldn't mess around in other people's business, but Mrs. Shears is a friend.
Not anymore, his father tells him. And anyway, Dad is really losing his patience at this point, so he very specifically tells Christopher a few things he's not allowed to do: namely, go around asking people about the dog, and anything involving "this ridiculous bloody detective game" (79.23).
His father makes him promise. So promise, he does.