Hemi Tokona was the grandfather of Matiu.
When Hemi was young, in 1953, he took the train to Auckland. The train crashed into the river, and Hemi helped rescue the survivors. He worked with a young man, not knowing it was his grandson. As they tried to rescue a trapped girl, Hemi was swept away by the waters. His wife and young daughter never learned what happened to him.
In 2004, Matiu sent himself back to 1953 to learn what happened to his grandfather. (PROSE: Water's Edge)