This chapter is the story of how Christopher's mother died two years ago. Here's the first part of that flashback:
Christopher arrives home from school one day and, when no one answers the door, he lets himself in with a key that's hidden outside.
His dad gets home, but neither of them knows where Christopher's mom is. After making some phone calls, his father says he has to leave the house for a while.
And boy does he mean a while: he's gone for a long time. Finally, he comes back and tells Christopher, "I'm afraid you won't be seeing your mother for a while" (43.7). It turns out she's in the hospital.
Christopher starts asking a lot of questions – because it's his mother and he wants to know what's wrong with her, of course, but also because he likes hospitals.
We learn that there's a problem with his mom's heart.
Christopher wants to bring her some food, because hospital food is gross. (He's right about that!) His father says he'll pick some things up for her at the grocery store and bring them to her along with the get well card Christopher is planning on making.