Answer and explanation:
The interior story, as the name suggests, is the story within another story. In this passage, the frame story is the speaker, Kevin, receiving a phone call from Matt, a friend he hasn't seen in almost ten years. The frame story is the one that involves the interior story. The interior story begins with the line '"Kevin, come inside!" my mother called.' This line separates the current moment from the moment the speaker is remembering - he and Matt playing outside; Kevin's mother calling him in. There is another story being told now, one about the past. But it is told as if it were happening at this moment. We have, from this moment on, an interior story.
An argument is sound if it is truthful and if it's valid. If all parts of it are valid and truthful, then the conclusion is also valid and truthful and therefore solid. That's why you should ask whether it is valid and true.
