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Not to worry: the day after those Black Days, Christopher sees five red cars in a row. It's a Super Good Day so he knows something good is going to happen.
After school, he goes to a store and runs into Mrs. Alexander. Maybe talking to her is the special thing that's going to happen on his Super Good Day.
Christopher does "some reasoning": his father made him promise not to do five specific things, and talking to Mrs. Alexander was not one of those specific things. So he figures it's okay to ask her about Mr. Shears.
Mrs. Alexander is hesitant to answer his questions, though. Christopher doesn't understand why, and he asks if Mr. Shears killed his (Christopher's) mother. (Answer: of course not.)
Christopher tells her that his mother had a heart attack and died, and Mrs. Alexander is shocked and surprised to hear this.
Then she says something else curious: "So you don't know?" (97.68). Christopher doesn't know what the heck she's talking about – and neither do we.
She says she really shouldn't be telling him anything, but suggests they take a walk together.
Christopher is nervous (because she's a stranger, after all) and excited (because she might know something about who killed Wellington).
Mrs. Alexander makes Christopher promise that he won't tell his father that she told him anything. He promises, so she spills the beans.
It turns out that, before she died, his mother "very good friends" with Mr. Shears (97.80). Eventually Christopher realizes she means that the two were having an affair.
He asks if this is why Mr. Shears left Mrs. Shears, and Mrs. Alexander says that it probably is.
Christopher isn't taking this well: he's frightened and says he's going home.