Published by Simon and Schuster 

Summary: Nick and Eryn are surprised and dismayed to learn one morning that their mother is getting married to Michael, her boyfriend of two years. To add to the upheaval, their mother tells them they’re moving into a new house.  Then she drops the real bombshell—unbeknownst to the twins, Michael has two children, Jackson and Ava.  Nick and Eryn have never met these two, or even heard of their existence.  They’re further mystified when they learn that each set of twins will spend alternate weeks at the house, so that they will never see each other.  Mom and Michael plan to elope, so there will be no meeting at the wedding.  Nick and Eryn are determined to learn the truth about their mysterious stepsiblings, little realizing that the truth will change everything they thought they knew about themselves, their families, and the world.  311 pages; ages 10-14.

Pros:  Fans of Haddix’s other science fiction books won’t be disappointed as she sets up a mysterious situation that unravels into more and more bizarre and unsettling revelations.  As in past stories, two ordinary kids are inadvertently thrust into a situation in which they basically have to save the world.  A cliffhanger ending all but guarantees at least one sequel.

Cons:  The adults in the story all seem somewhat stiff, with forced conversational skills.  You’ll have to read about half the book to figure out why.