4 ideas
A young man and a young woman live in adjoining buildings of an unnamed city. After a brief chance meeting they exchange phone numbers on scraps of paper that the rain washes illegible, and the rest of the book follows them repeatedly passing within metres of each other without meeting.
A trivial default, such as which way you turn when you step outside, compounds day after day into a fixed map of who you can and cannot meet. Seeing routines this way shows that much of what feels like destiny or bad luck is the cumulative geometry of small unexamined choices.
Dense cities put people physically close while keeping them socially apart: shared walls, parallel commutes, and the same parks do not produce encounters. Nearness is not connection, and a person's likeliest match may be the neighbour they have never seen because separate routines channel everyone past each other.
Whether a chance meeting becomes a relationship often depends on a flimsy physical token, such as a number on wet paper. When that thread breaks, the encounter reverts to pure chance, and the people involved have no way to repair the loss except by waiting for luck to repeat itself.