In Chinese class, students from higher grades wrote poems revealing their tender adolescent love. For music class, Hou played guitar.

"Yingying loves music," Hou told an audience who were out for Zhang in the concert one day before Christensen was arrested. "I wrote many songs for her, and this is her favorite: Dream Like A Child."

On that hot summer day in Beijing in 2018, after showing Shi the place where Zhang had lived, Hou was about to leave. As he was walking away, Hou heard a thumping sound, and looked back to see a bird, seized by apparent pain, flapping and thrashing itself on the pavement in a futile effort to fly.

"I don't know where the bird came from-it seemed that she had dropped from somewhere quite high and had severely injured herself," says Hou, who, under Shi's camera, stooped down and put his hand gently over the little creature.

The bird soon became quiet. When Hou lifted his hand moments later, the bird's life was gone.

Hou put the bird into the small hole he had dug out on the grassy ground with a stone chip, covered it with dirt, smoothed the earth, and then they left. Nothing seemed to have happened.

"I wasn't thinking much at that moment," Hou says."But as I was watching the movie, it became so clear that it was Yingying. She came to say goodbye, and to give me what I'd always wanteda proper burial for her."