A cabby picks up a fare and takes her around town only to find she doesn't have the money to pay. O. Henry admits part of the reason for this may be the physical separation caused by how the hansom is built. Even if the cabby were interested in looking at their rider's face, the structure of the hansom makes it pretty much impossible. But, as it happens in this story, the lack of contact can lead to some interesting situations.
But who the fare turns out to be makes for quite a twist ending.
O. Henry's short stories are well known for their wit, wordplay, warm characterization and clever twist endings.