Set before cellphones were a thing, a working man one day finds a young girl on the other side of his front door only to learn that his father may have had bastard lovechild that's come a knocking on their home. Her mother had died recently and shared with her the identities of her possible fathers so that she may have some closure on her parentage.
Unfortunately for her, his parents had passed years ago and he obviously heard nothing of a half sister from either of them. He's shocked at first but ultimately glad to hear he may not be alone so he sits down with her to see if they can figure out anything conclusive from his dad's old journals and datebooks. They find that there was in fact a business trip that just barely fit right within the timeline but can't conclusively say one way or another. Moreover, his father wasn't the type to fool around so things don't look too promising for her.
He then suggests they do one of them borderline sci-fi DNA tests he's heard about and she rejects it on the basis that she doesn't have the money for it. He learns that she only just graduated from highschool a few months back and that though she has enough money to get by on her own for a decent while, she can't afford to spend any of it on anything but necessities. On top of that, she still has another half dozen names to check through so thanks him for his time and will get back to him on how things turn out.
He lets her go on her way but fate works its magic and days later when he discovers that "a decent while" had already come and gone for her. Not able to leave the girl to her wretched living arrangements, he takes the girl by her hand and declares that she'll be keeping him company in his big empty house from now on. She protests but he insists and gets her to give in by saying that it's a maybe big brother's job to take care of his maybe little sister.