Identical twins have been swapping places for centuries now, from the Man in the Iron Mask through to The Parent Trap, but the twins in Beside Myself don’t exactly swap – it’s more like one performs a hostile takeover on the other.

Helen and Ellie are identical, although Helen is the leader and silly, slow Ellie is the oppressed follower. The school, their friends and even their mother all have a slightly lower expectation of Ellie, just waiting for her to mess up. Then one day Helen suggests they swap places and then Ellie refuses to swap back.

Before she knows it, Helen is forced into the role of secondary twin – rejected and forced into the background while Ellie (or Hellie as her bitter twin comes to call her) gains confidence and grows. There follows a dark and gripping tale of addiction, mental illness and the strange bond between sisters who hate each other, but still share a link which neither of them can break.

How much of our identities is made up by the judgements and opinions of others?

Like many books being pitched as part of Thriller Season this year, it’s not really strictly speaking a thriller. There are no twists and turns or ghastly murders – but it is addictive reading with a few revelations along the way. And although the central idea seems a little silly (Surely someone would notice. Don’t twins have different moles and stuff?*) the idea that such a thing could happen, and the havoc it could wreak on someone’s life and sense of identity is nothing short of compelling. That’s what keeps you reading on.

It’s a great one for reading groups as it throws up so many questions. Would Helen have continued to be “perfect” if the swap hadn’t happened, or would the flaws in her personality have come out anyway? And how was it that Ellie could suddenly lose her clumsiness, her untidiness by taking on Helen’s mantle? In short: How much of our identities is made up by the judgements and opinions of others?

For anyone who ever had a sibling and saw small differences in their treatment, the question is irresistible, and the novel itself is eminently readable. So although the central premise is old as the hills and there are a few cracks in it besides, it’s still a fresh, entertaining take which is definitely worth reading.

*There is a partial explanation but it would involve spoilers so I’m keeping schtum.

Beside Myself by Ann Morgan is out now.

Other Thriller Season reviews include…

The Widow by Fiona Barton

The Darkest Secret by Alex Marwood

Viral by Helen Fitzgerald