Starting in the year 2018, every person born has an identical twin. What was dubbed the mysterious Set Mutation is now, twenty years later, a worldwide phenomenon. Penny and Sam, two 16 year old twins, have never understood why they are different from one another. While all the other sets of twins grow to be more alike, in every way, Penny and Sam struggle to fit in—they aren’t naturally the same person. Soon they will find out how deadly their secret really is.