"Vastra and Strax and Jenny? Oh no, we don't need to bother them. Trust me." 
 
Marlowe Hapworth is found dead in his locked study, killed by an unknown assailant. This is a case for the Great Detective, Madame Vastra. 

Rick Bellamy, bare-knuckle boxer, has the life drawn out of him by a figure dressed as an undertaker. This angers Strax the Sontaran. 

The Carnival of Curiosities, a collection of bizarre and fascinating sideshows and performers. This is where Jenny Flint looks for answers.      

How are these things connected? And what does Orestes Milton, rich industrialist, have to do with it all? This is where the Doctor and Clara come in. The Doctor and his friends find themselves thrust into a world where nothing and no one are what they seem. Can they unravel the truth before the most dangerous weapon ever developed is unleashed on London?