Fourteen-year-old Amy Cahill and her younger brother, Dan, head to the Land Down Under todiscover what their own mother and father knew about the hunt for the 39 Clues. But following in their parents' footsteps brings up lost memories for Amy so awful that she can't share them... even with Dan. Haunted by the ghosts of their past, chased by deadly competitors, Dan and Amy can't see who is an enemy and who is a friend. Their blindness leads to a terrible mistake... and the death of a hidden ally.
Amy and Dan decide to head to Austrailia after they get a faint hint in the previous book, there they meet their dads cousin Shep, a surfing bum who knows nothing about the 39 Clues, but ended up ferrying Amy's parents around the outback awhile back, oblivious to the real motive behind the flight.
In Austrailia Dan and Amy meet the Kabra's evil mother, Isabel Kabra. Later did they realize that it was her that took Graces necklace off of Amy, and that she lies really well. Isabel had threatened to throw Amy to the sharks, and almost suceeded, if Hamilton Holt hadn't come to the rescue on a paraglider.
Irina constantly tries to warn Amy that she is not the threat at hand, and that Isabel is more evil than she might think. Amy is bombarded with stories about who really killed her parents, and that it wasn't an accident like it was claimed to be. Isabel blames Irina, and Irina blames Isabel. Amy is struck with nightmares about what really happened, and tries to find the answer without alerting her brother, Dan.
Towards the end of the book, Amy has a dream/nightmare where she finds out the true killer of her parents.
"There's a reason I didn't tell you about Mom and Dad being murdered," she said haltingly. "A 'nd it wasn't because I couldn't trust you. It was because I remembered something I did. I didn't want you to know. I-I didn't want you to blame me"
He shot her a questioning look.
"That night... the night of the fire... I was still awake when the strangers came. I heard them downstairs. I listened at the door. They had been asking Mom and Dad where they'd been. They asked over and over." Amy paused, and then the words rushed out. "I was scared. S-so I ran inside the room. A woman picked me up. Isabel. She talked about the teddy bears on my night-gown and I corrected her. I said they were koalas. So that's how they all knew."
Irina had been right. Isabel killed their parents, and tried to get rid of Amy and Dan the same way. Irina had come and saved Dan, Amy, and Alistair in Kartatu, and had paid for it with her life.