The grandson of the infamous Dr Jekyll returns to London from Ceylon in the 1930s when he receives an inheritance. As he delves into his family's mysterious past, he does not realise he is being pursued by members of a secret society - while a ruthless spymaster intends to use him as bait for a trap.
Robert survives an attempt on his life, but is not sure how. He follows the advice given to him by Lily and Max, and goes looking for answers in his grandfather's house. His search leads him back to the East End, where he was attacked, and an unexpected encounter at the Empire Hall. Meanwhile, Ravi has to face both the colonial police and the undead to bring Robert urgent news from Ceylon.
Robert becomes determined to rid himself of his alter ego, and as he falls in love with Lily, he resolves to keep Hyde a secret from her. In desperation, he takes some of the potions he finds in his grandfather's laboratory. Garson tells Robert that his grandmother may still be alive, and he sets off in search of her - but Captain Dance is also trying to track her down.
Wracked with guilt after failing to get to his grandmother Maggie before Captain Dance's men, Robert, Lily and Garson work tirelessly to cure her of sepsis. Garson makes the chance discovery that Hyde's blood has antibiotic properties. Dance and Bulstrode play cat and mouse in their pursuit of Hyde because Dance believes that only Hyde can help him resurrect the demonic Lord Trash. Robert risks everything by turning into Hyde, but will he be able to save Maggie?
Robert discovers he has some distant relations, so he and Max travel to the remote village where they live in the hope they will be able to tell him more about the Jekyll curse. On their arrival, they discover several mysterious deaths have occurred recently and the locals believe a legendary demonic dog is responsible. Robert discovers an ancient book that seems to reveal a connection between his relatives and Tenebrae, and suspects they are dealing with a shape-shifter.
Part one of two. Robert is tormented by the recent tragedies that have befallen him, causing him to lose control of his transformations. Bulstrode reveals that military intelligence is holding Ravi prisoner, but will release him in return for Robert's help in investigating a series of strange killings. Robert takes on the case, and joins forces a vigilante also on the murderer's trail - but when the truth behind the slaughter emerges, it leads to his new ally becoming a dangerous threat that Bulstrode wants eliminated.
Part two of two. Robert goes in search of his former ally, the vigilante Spring Heeled Jack, and the mysterious parasite that has taken control of him. However, the creature abandons its host and take over Ravi, who now faces either a painful death when it is finished with him or being killed by MIO agents in an attempt to destroy the infection. Robert tries to find a way to save his brother, while also using the parasite as bait to lure Bulstrode into a trap and rid himself of MIO once and for all.
Robert has no wish to find the monster that is killing wealthy young men, but the trail is too compelling to resist. It leads him to a fight with a beautiful vampire who shocks him with the news that she's his sister, Olalla. She also tells him Dance is alive and being rebuilt by eel creatures. Robert is suspicious of a trap and sets a test - if she can turn into a Jekyll he'll believe her.
Bulstrode searches London for Robert, but does not realise he has fled into the countryside to hide out at the Jezequiel house, taking Ravi and Olalla with him. To his surprise, they are later joined by Lily, who claims to have guessed where he would go. However, Ravi begins to suspect the truth - that what they think is their friend is actually a shape-changing monster known as an Incubus. As the others fall under the monster's spell, it appears that the creature has the ability to restore Lord Trash to the land of the living.
Robert frantically searches for Lily, while Captain Dance is restored when his men are able to harness the power of the Incubus, and he prepares to put his final plan into action. He launches an all-out attack on MIO headquarters to steal the jar containing Lord Trash's heart and restore the nightmarish creature to the land of the living. Robert is devastated to learn that one of his closest friends has been lying to him all along, but fresh hope emerges when he discovers there is a way to destroy the heart once and for all.