From the ABC website-
“On a winter morning in June 1980, Maria James turned to her son Mark and looked him in the eyes. “If anything happens to me, look after your brother.” Later that day she was found stabbed to death at the back of her suburban Melbourne bookshop.”
The four episodes plus a mini bonus episode can be listened to here-
There are also five written articles available at the above linked page. They contain additional information to the audio episodes for those keen to understand what happened.