Navel String

It was a bond that, if severed, would result in death. Mariko gives birth to a son, Masato, but they're afflicted by a strange disease that will cause them to die of shock if the umbilical cord that connects them is cut. What are the concepts of "mother and child," "father and child," and "family"? This is a story of "the end of connection." (Source: Shogakukan, translated)