The new series sees Gemma 18 months on from where we left her in series two, and life has settled into a familiar rhythm. Her baby, Sadie, is now in nursery, Gemma's been promoted at the salon and Malcolm has just ordered a fancy new washing machine to optimise the laundry. Things might finally be on an even keel for the Moscrips - so maybe there's space for Gemma to finally find love? However, it doesn't take long for chaotic 'old Gemma' to rear her head; a romantic encounter escalates into a whole heap of problems, and there's only one person she can turn to - her dad, who might finally have become the grown-up she always knew he could be.

故事接着第一季结局讲起，谜题大师约翰·“路德维希”·泰勒如今已成为剑桥警察局的犯罪现场顾问，专攻“不可能”案件。无需再伪装成兄弟，他比以往任何时候都更坦然展现“路德维希”本色——在解谜上大放异彩，其他方面却一塌糊涂。
但约翰的孪生兄弟詹姆斯至今下落不明。现在约翰成为了警局的正式雇员，因此被禁止动用任何警力资源寻找兄弟，或调查兄弟生前究竟在追查什么案件。
当然，约翰不会就此罢休，露西，约翰失踪兄弟的妻子，同样不会停歇。一个谜团有待破解，身为丈夫和父亲的詹姆斯必须平安归来。一场假面舞会或许落幕，但新的假面舞会才刚刚开始。