After fighting for a seat on their morning commute, Sally and Carl begin talking and suddenly their daily train journey becomes a lot more interesting. Carl is happily married, Sally's engaged - where's the harm? Yet they find themselves increasingly drawn to each other, and as their friendship grows to flirtation they refuse to admit - to each other or themselves - that a line might be crossed. The consequences of discovery will be life-changing, catastrophic perhaps, and yet they can't help falling in love.