What is the main conflict in The Child in Time by Ian McEwan?

Stephen Lewis wakes on a Saturday morning and plans to take his three-year-old daughter Kate to the market for their morning's shopping. He leaves his wife Julie asleep in bed. At the market, Stephen takes Kate from the shopping cart and begins unloading the groceries to check out. When Stephen turns to check on Kate, she is not here. Thinking she has stepped back from the cart, he pushes it out of the way and searches for her, but cannot find her. A search of the street is unfruitful as is a search of the store's loading dock. Other shoppers get involved and the police arrive, but Kate is not found. Stephen then has to go home alone to tell Julie the news.