- Martha is closer than ever to achieving her evil goal. Rob and Fran attempt to stop her, and are held prisoner. Nell launches a desperate last-ditch bid to rescue Luke from the cave, preventing Martha's ritual from unleashing destruction.
- The attacker misses Nell with the knife, striking the fifth barrel instead. She finds a loose rock on the tunnel walls and hits the diver with it, disorienting them and gaining a chance to escape from the tight space in the process. Loosened by the knife cut, the barrel falls on the attacker, trapping them underwater. Nell rises back up to the surface to see Rob and Luke; whilst the former assists her with the scuba gear, she breathlessly tells them her realisation that the attacker is 'Erlingham' - and that she thinks he's dead.
Tom and Nell go back to the harbour to investigate the belongings left by 'Erlingham'. They discover his real identity, Kenneth Runceford, and his job as a 'field operations executive' for the Drell Mont corporation, as well as a laptop with the disk stolen from Fran in his car's boot. Meanwhile, Fran and Rob return Luke to the mansion. Martha asks where he was, and Fran tacitly says they found him in the harbour. They are interrupted by Tiarks, informing them that a body has washed up in the cove and the coastguard requires identification.
Whilst observing new information obtained from Runceford's belongings on the Eye of the Storm, Tom discovers that his company had a contract with Middle Eastern countries to reprocess and make their chemical weapons safer. It transpired that some were so lethal and deadly that they could not do anything. They instead chose to use the 1987 storm as an excuse to dump them, farming the job out to a smaller company based in Felixstowe. Tom says the substance in the fifth barrel appears to be a binary chemical, explaining how they work to Nell; if another, much simpler element is combined, even if just 'school chemistry stuff', the weapon is activated. They remain apprehensive about it.
Elsewhere in Montliskeard Bay, Rob accompanies Tiarks, Howie and Pryce as they identify the body. Tiarks tells the coastguard that it is Barnes, who was staying as a guest up at the mansion, and fakes his grievance for him. As the henchmen leave, Pryce tries to tell Rob that he has to take Luke away from Martha - but they are interrupted again by Tiarks, who forcefully commands them that it is time to go back up to Montliskeard Place.
Fran asks how Barnes' death happened in the study, and Martha lies that it was a terrible accident, claiming that he slipped on the cliff edge and fell into the sea. She immediately changes the subject, telling everyone that before his death, Barnes supposedly gave her his amethyst to go on with the ceremony. A distant, disassociated Luke agrees, urging her that it must happen. Confused by his sudden change of demeanour, Rob protests Luke's zombie-like state, and accuses Martha of poisoning him to her chagrin. Fran tries to let Luke 'have his say', to which he responds by simply repeating himself; obvious that he's unwell, she then offers to take the boy to a doctor. Martha asks how she will do that, and her two main henchmen make their intimidating presence felt, effectively holding Fran and Rob hostage.
Tom continues to study the documentation from Runceford late at night. He awakens Nell from her sleep to try and find out the markings that she saw on the fifth barrel in the tunnel. Though annoyed, she tells her father that it had a yellow checker board pattern. He checks back on the laptop, and realises the exact chemical in the barrel was 'CGT' - a highly toxic binary nerve gas, withdrawn from NATO issue due to its dangerousness. Originally intended for special naval and amphibious military operations, it is said to have excellent dissipation in water, and is triggered by simple compounds of potassium, even proving his previous 'school chemistry stuff' explanation right.
Meanwhile at the mansion, Fran and Rob enter Luke's room, worried about his health and intending to get him out of there. They try to wake him up, but he is unresponsive. Rob keeps an eye on him, and Fran goes into Martha's empty study in an attempt to find out what she has been drugging him with. She finds mercury, antimony, and saltpeter in the cupboard, but alerts her and Tiarks after accidentally smashing bottles. Martha insists she knows Luke is ill, claiming he is on prescribed medicine, and mocks her for thinking she was giving him saltpeter. She explains to Fran they follow the tradition of the alchemists, and saltpeter's ancient use as gunpowder.
Rob overhears Martha, and whilst she is still distracted with Fran, he sneaks out of the mansion to tell Tom and Fran his concerns about her talk of saltpeter and 'greek fire'. Tom quickly recognises its modern definition as potassium nitrate, and tells them that if it gets anywhere near the tunnel and CGT barrel during her 'ceremony', Martha could kill all life on the entire south coast of England with the resultant vast fire. Nell and her father hurriedly get ready to try and stop her while they still can, with the ceremony due to start in just a few hours. Rob goes back up to the mansion intending to enact his own plan of action, despite Nell's worries for him.
At dawn, Tiarks gets Luke ready, draping him in a red cape. Martha stands before the portrait painting of Gilbert in her study with Pryce, Rob, and Fran behind her, proclaiming that it is his day. They leave the mansion through the dilapidated underground stairway, accessing a dark tunnel to the cave through the locked door. Martha leads Fran, Rob, and Luke down it. At the same time, Tom and Nell get to the underwater cave entrance on a dinghy. Tom has reservations about Nell's insistence that she must go alone, but she concerns they are wasting time. Martha reaches the candle-lit cave with the others, making final preparations by wearing her own cape.
Nell launches the desperate bid to save Luke and the coast alone, diving into the underwater tunnel once more and leaving Tom waiting for her above. Realising that Runceford's body and the barrel block her, she has no choice but to take off some of her scuba gear to get through.
In the cave, the ceremony is underway; Martha pours her elements into the the water, whilst reciting lines of an ancient alchemy text with Luke. The fifth stone in the armillary sphere begins to shine brightly, whilst Nell tries to stay hidden in the underwater entrance. Rob sees Martha ignite the saltpeter and runs towards her in an attempt to fight, but she pushes him into the water. Fran argues the deaths she will cause to no avail, and Martha throws the fire, saying it is her master's will.
The saltpeter basin spins down into the water. Nell suddenly vaults out of it, catching the bowl and interrupting the ceremony dramatically. She throws it back at Rob, urging him to put the fire out and leave the cave before it floods. He complies by using his shirt, as a rumble grows around the cave. Water crashes down from several crevices, smashing all of the candles and startling Luke out of his trance, who Martha holds onto tightly. Nell tells Rob and Fran to get out through where they came, as she will get Luke.
Rob and Fran escape. Still clutching Luke, Martha looks around in horror. Nell fights to get her off of the boy, and in the struggle, Martha accidentally traps her arm in the armillary sphere, leaving her to drown screaming as Matthew and Barnes did before. With the rest of the cave rapidly filling up around them, Nell takes Luke and instructs him to take the biggest breath he can with her on the count of three, promising he will be safe. They hold onto each other and swim up the tunnel. Tom awaits his daughter on the surface nervously, but eventually sees her triumph as she comes out of the water with Luke, bringing the dinghy over to help them.
Several days pass, and the Frewens come to visit a recovering Luke in hospital. The doctor tells them that his guardian's drugging induced blindness, but they have never had a case like him, and are unsure if he will be able to see. Tom and Nell enter the room to find Luke asleep, with Fran and Rob joining them. In the background, Fran's report on the discovery of the fifth barrel is airing. As she closes the report by crediting Tom and Nell's efforts, Luke wakes up, and tells Fran she is prettier in person than she is on TV.
Overjoyed, everyone realises that he can now see again. Nell and Rob introduce themselves, and Nell introduces her father, whom Luke had only briefly met. He jokes that Tom looks 'quite old', to Fran's amusement and his mock displeasure, and Nell hugs them.
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