He nearly jumps out of his skin when she opens the door. A small part of him had thought, yes, she could be home, but he didn't want to get his hopes up. Yet there she is. Looking as beautiful as ever. His nerves are shot; he forgets why he came here for. She looks at him, says 'yes?' She doesn't know him.
He knows her. So he says he's looking for her dad, and he's taken to the porch. Immediately, his nerves escalate as he realizes he's there at a bad time. Small talk, an introduction, and he finds he can laugh off the nervousness as he hands in his dissertation. He's fully aware of Catherine. He wants to look over, take her in. When they talk, he finds he's caught between never wanting to stop and crawling under a rock lest he continuing sounding like the geek he knows he is.
He wants to kick himself for making her feel embarrassed. And then he shuts up, and takes in wonder that is father and daughter, captivated by the way they talk with one another; and a little envious.
When she invites him to go to dinner with them, his heart jumps into his throat. He looks into her eyes, and knows he'll only make a fool of himself; a lame excuse - "I have plans" when sitting around with his fellow would be mathematicians, drinking and rambling off prime numbers could be called nothing of the sort. Still, he doesn't imagine the small, inexplicable moment that passes between them when she urges him to come along.
He nearly stumbles as he leaves the house, cursing himself for not going along, for not spending time with Catherine like he's always wanted. And now she's going to Northwestern and he's missed his chance like the idiot he is.
He gets in his car, and remembers the first day he saw her; bouncing down the hall to her father's office, a smile on her face so huge it could light up the world. He wonders what she was happy about that day. And the other times, when she visited, and he'd catch glimpses of her. He'd fight with himself to go in, just a casual visit after all, talking to his advisor, no big deal.
But he'd chicken out, and she'd leave looking sad and worried. And, of course, he knew about Robert, knew the man would only get worse. He wanted to go over to her, to say something charming. But he isn't that guy, and now, once more, he's missed his chance. He has to meet with Robert next week; one last trip to his office. He drives away, hoping he can catch one final glimpse of Catherine before she goes.