After Dark is a novel set during one night in Tokyo. It follows Mari Asai, a young student who spends the night in a café and meets several strangers whose lives become unexpectedly connected. At the same time, her sister Eri remains in a mysterious deep sleep, unable to wake up.
As the night unfolds, Mari becomes involved in helping people she meets, while strange and dreamlike events blur the line between reality and fantasy. Through these encounters, she begins to understand herself and her relationship with her sister more deeply.