What is the main conflict in How to Live Safely in a Science Fictional Universe: A Novel by Charles Yu?

Charles Yu's debut novel, How to Life Safely in a Science Fictional Universe, could be described as a story about contemporary family life disguised as science fiction. It concerns a young man who has spent most of the past decade in a small time machine in his job as a time machine repairman. He makes calls on people who have rented time machines for recreational purposes but have become stuck in time and must be rescued by him.