Bringing together four crime writers to write one thriller could be a disaster. But in the case of The Understudy, this quartet pull it off with aplomb, while clearly having a whale of a time. The set-up verges on the melodramatic, but that’s all part of the fun. Four teenage friends attend the exclusive Orla Flynn Academy for the Performing Arts. When a new girl joins, threatening things start happening to the group: is the new girl responsible, or is one of the friends hiding something? Told from the perspectives of their four mothers, and through glimpses of the girls’ Snapchat exchanges, this is as scathing, astute and tangled as a teenager’s friendships.