When will Daddy be back? This question has been troubling 7-year-old Wu Wei for a long time. Since a year ago, his father has not come home. In response to Wu Wei's inquiries When asked, mom always avoids talking about it. His only solace is a small handmade tin train left to him by his dad. And what makes Wu Wei feel most at risk is that the Mom seems to be close to a new uncle. The new uncle gave him a new train, and the train became a nightmare in Wu Wei's life. Wei even transforms it into a monster. Faced with the huge variables in his life, Wu Wei decides to take his beloved little train and set off to the deepest place to bring his father Wake up.