Book Series · 2 Books

Under Their Skin Books

In order from Under Their Skin to In Over Their Heads.

Margaret Peterson Haddix's two-book science-fiction mystery follows twins whose search for their hidden step-siblings uncovers a secret much larger than their new family.

Reviewed by the Bookroo Editors Updated August 13, 2026

Reading order

How to Read Under Their Skin

Read Under Their Skin first, followed by In Over Their Heads. The second book begins after the twins learn the family's central secret and brings their continuing conflict to its conclusion.

Sources: Simon & Schuster's official series page.

For parents

What Parents Should Know

The mystery begins with family secrecy and children disobeying adults to investigate. It develops into science-fiction danger involving artificial intelligence, hidden identities, surveillance, capture, weapons, and threats to both human and robotic lives.

The action is suspenseful but not graphic. Questions about what makes someone human, whether safety justifies secrecy, and how families include people who seem different give parents useful discussion topics.

Where the story begins

First Book in the Series

Sale price

$1797

Book #1 · Chapter Book · Ages 8–12 · 2016

Under Their Skin

By Margaret Peterson Haddix

Twins uncover hidden family secrets that could change everything.

320 pages· 55,573 words· Est. 7.5 hour read

$18.99Save 5%

Hardcover

Summary: Twelve-year-old twins Nick and Eryn are thrust into a mystery when their mom remarries, and they learn about secretive stepsiblings they must never meet. As they dig deeper, they uncover unsettling truths that could put their lives in danger. Why are these siblings hidden, and what secrets is their new family concealing? With their world turned upside down, Nick and Eryn must navigate a web of lies to uncover the truth. Will they unravel the mystery before it's too late?

In reading order

Under Their Skin Series

2 books · Published 2016–2017

#1 in Series

Sale price

$1797

Under Their Skin

Chapter Book · Ages 8–12 · 2016

Twins uncover hidden family secrets that could change everything.

#2 in Series

Sale price

$1797

In Over Their Heads

Chapter Book · Ages 9–12 · 2017

Twins race to save humanity without losing their newfound family.

Good to know

Common Questions

How many books are in the Under Their Skin series?

There are 2 books in the Under Their Skin series. It began with Under Their Skin in 2016, and the newest book, In Over Their Heads, was published in 2017.