Nyssa approaches her patient and assures him there’s no need to be afraid as she’s here to help. She tells him he’s not going to die, but he doesn’t believe her as he knows the disease is terminal. She tells him she had the disease once, but she survived and now she’s trying to find a cure. When he insists no one has ever come back after having the treatment, Nyssa explains that she did, and she chose to stay. She decides to tell him her story, after which she says he can make a choice.

Her story is from the time when she was travelling in the TARDIS with the Doctor, Tegan and Adric. Her homeworld was destroyed and everyone she knew and loved was wiped out in an instant. On Traken the people believed they could overcome evil by ignoring it, but there are some evils you just have to face. On one of their journeys, something happened to Nyssa that the Doctor never found out about. She went beyond death and she believes her experiences will enable her to save the patient.

Nyssa hadn’t been sleeping well and was being haunted by dreams of black velvet gloves tightening around her throat. When she joined the others in the TARDIS control room she discovered the ship had been forced to land against its wishes and the dimensional alarm was sounding. The scanner showed they’d were in a long, dark corridor of metal and although the Doctor advised discretion, he dashed outside after checking the instruments.

The corridor was in a state of devastation, full of twisted metal with wiring conduits hanging from the walls. They were evidently on a crippled ship as some of the crew were lying dead on the floor, crushed when the interior collapsed. The Doctor said they must have arrived in the aftermath of a battle and he pointed to an area where the corridor was open to space. Beyond the wall, they could see they were in orbit above an orange planet, but the area was littered with floating debris, wrecked ships and more dead bodies. The ship they were on was clearly disabled, yet they were still able to breathe. Nyssa saw a figure moving out of the corner of her eye and the Doctor rushed off down the tunnel in the hope that it was a survivor. The others followed him to a dead end where the corridor was blocked by a mass of steel, and although they appeared to be alone, the Doctor was sure something else was there with them. He pointed out that someone must have gone to a lot of trouble to give this ship an atmosphere and he wanted to find out what had forced the TARDIS to land here.

Suddenly two shadows detached themselves from the dark walls and converged on the group. Adric tried to sidestep one of the figures, but it raised its arm and a metallic spear extended to point at his throat. The figures were both large and each was wrapped in a complicated mechanism that breathed in and out. They were draped in the remains of dead creatures and they gave off a smell of ammonia. Their heads were skulls with a thin covering of flesh stretched across them, with a metal grid for a mouth. The Doctor tried to calm the situation down, but one of the creatures hissed at him and declared that both he and Adric were dead. The second creature said they were Dar Traders, a race that salvaged the dead but had no claim on the living. The Doctor had never heard of them, but they explained there was a war in this system and they’d followed it to the planet where they intended to trade. They said they take the flesh of the dead and give their knowledge in return. Tegan was horrified, but the Doctor thought it sounded fascinating. He asked what the creature meant when it said he and Adric were dead and it told him they both had shifting death signatures. The Doctor assured them they were very much alive, but the Traders insisted they go back with them to their ship where they could be catalogued. If it turned out they were not dead, they would be free to leave.

The Doctor said he’d come here in search of a dimensional anomaly, but Nyssa had a terrible sense of premonition which gave her a headache. She saw an image of herself lying in Tegan’s arms, with leaves blowing all around her. Just then a great explosion erupted near the gaping hole in the hull and they saw another spaceship passing close by. It was on fire and a second explosion caused it to shatter completely and break up into a thousand pieces. The Doctor urged everyone to run back to the TARDIS, but there was no time to react and the corridor was soon filled with the shrapnel of burning metal. As the entire ship rolled and shifted, everyone was thrown to the floor, but incredibly the Dar Traders’ force shield held and the atmosphere remained intact. When the group recovered, they noticed one of the Traders had been blown out into space and the Doctor had disappeared. Adric pointed into the gap and they could see the second half of their corridor spinning away into the distance, having been broken free by the explosion. Slumped against the wall, the Doctor could be seen unconscious.

The surviving Dar Trader said Tegan, Nyssa and Adric must go back with him to his vessel as it was no longer safe for them to remain aboard this ship. The corridor was beginning to fall apart, so the group realised they had no choice. The Doctor slowly started to recover and clambered to his feet. He looked across to his companions and began miming instructions to them. Nyssa realised he intended to go back to the TARDIS and would come to collect them. The Trader told them that wouldn’t be necessary and it produced a square of metal from its chest which unfolded into the shape of a door. In the doorway was a mirror and on the other side they could see an entirely different location. The Trader warned them it could no longer maintain a safe atmosphere here and it ordered them to step through. Adric obliged willingly and vanished with a shimmer, but Tegan refused to move and the Trader extended its spear again and prodded her through. One by one they moved inside the doorway and disappeared.

The journey was instantaneous, but Nyssa suspected they’d travelled a great distance. Inside the Dar Traders’ ship, the walls were covered with bits of dead bodies, including faces, and there was a harsh smell of formaldehyde in the air. Tegan thought the displays were ghoulish, but Adric criticised her for not understanding other races’ cultures. The Trader said his race wished to experience the moment of death as closely as possible as they believed it to be beautiful. It told them it could sense the Doctor on the planet below, but before it would let them join him, it said they had to undergo cataloguing. It warned them that if they were designated as salvage, then its people could claim them.

Despite their objections, the Trader herded Tegan, Nyssa and Adric down a series of corridors filled with more body parts, either displayed out of reverence or utilised to shore up the damaged ship. Nyssa’s headache was becoming more intense and she wondered whether the Traders were able to see into people‘s futures to the moment of their deaths. She remembered her premonition and it occurred to her that she might have tapped into that ability. Eventually they arrived at a large chamber containing glass-fronted stasis units, stacked up on top of each other until they disappeared up into the darkness. The Trader explained that these units contained the Sleepers, Traders who seek to experience the threshold, the place between life and death. There was something in the idea that appealed to Nyssa as it combined her scientific and spiritual aspects. The Trader ordered Nyssa to enter the cabinet first and although she was anxious, she also wanted to show some trust in them as she was sure they didn‘t mean to harm them. Inside, she felt the interior grip around her body, then a white light began to pulse. She began to feel pain, but the Trader said this was the only way they could measure if she was alive. The readings were currently at 40%, but the Traders had encountered dead people before who could resist a reading of 70%, so it increased the setting.

In a haze of pain, Nyssa once again saw a vision of herself being held by Tegan in a carpet of leaves. Suddenly the cabinet opened and Tegan and Adric pulled her out. The Trader had identified Nyssa as a Trakenite and said that although death was attracted to her, she had the ability to master it and calcify it. It said she could be useful to its people and suggested they trade. Nyssa realised trading was merely a business to them and they’d aborted her cataloguing because she had some bargaining power. She didn’t want Adric and Tegan go through what she’d just experienced, so she agreed to trade with it if it’d allow her to vouch for them. The Trader insisted that both her friends had been “touched” by death and that death followed Adric particularly closely, but it agreed to let her claim salvage over them. Nyssa asked it to take them to the Doctor first, but the Trader wanted something else in return first.

The Trader took the group towards the centre of the chamber where he operated three rods which began pulling a black cabinet from beneath the floor. The cabinet had runes carved into the front and it was pitted with holes and covered in spikes. The Trader explained that it had been taken as salvage from the planet below and it had strange properties. Other Traders had entered it and died, then they came back physically wasted and in need of augmentation. They spoke of the threshold, but they were different from when they entered and their death signature was wrong. Although Traders believed the threshold to be beautiful, they always wanted to come back to the corporeal Universe. Tegan realised they wanted to experience death, by “trading” and “collecting” it, in order to better understand it, but they didn’t want to actually die. Nyssa began to see the benefits of their way of life. The Traders had willingly taken death into themselves as an art, but their survival and build-up of immunity meant they could probably generate a cure for every disease or ailment that existed.

As Adric collapsed, Nyssa remembered the Dar Trader telling her death followed the boy closely. It occurred to her that the creature must have known he would die here. She wished the Doctor were here to help…

The patient tells Nyssa she used to depend too much on the Doctor as he probably wouldn’t have been able to save Adric even if he had been there. Nyssa reveals that, in fact, Adric didn’t die on the Dar Trader ship. There was another death waiting for him in the near future and that’s what the Traders had seen. The patient thinks Nyssa’s cure must somehow be connected to the man in the cabinet, but she tells him it’s more complicated than that.

Adric had been stabbed through the heart - or rather, where the heart would have been in any normal humanoid - but being Alzarian, his heart was on the other side of his chest and he‘d been saved by a simple quirk of biology. There was too much blood for Nyssa to see how bad the wound was, but miraculously it seemed to be healing already. Adric was in shock, but he was still able to reassure his friends that Alzarians heal very fast. The Trader restrained the man in the cabinet by pressing his spear against his neck and Nyssa demanded to know why he’d attacked the boy. The man introduced himself as Damasin Hyde and explained that he was an assassin for hire. When he revived from his sleep, he believed Adric to be his quarry, but he assured them the blade did not strike the boy too deep. He said the machine kept him alive while he was being transported between commissions, but the Trader knew it was more than just a suspension cabinet and said it has unusual properties. Damasin claimed that it was made of Dwarf Star Alloy as an extra layer of protection, but he soon became tired of the constant questioning and demonstrated how he could have escaped from the Trader’s spear at any time.

The assassin realised for the first time that he was no longer aboard his own craft and the Trader said his people had removed the cabinet from a crashed spaceship they found on the planet below. Damasin became angry, but the Trader pointed out there were no life signs aboard when they scanned it. The cabinet was damaged so Damasin demanded to be taken back to his ship so he could fix it and be on his way. Adric told Nyssa to go with him down to the planet to find the Doctor, but the mention of the Doctor‘s name caused Damasin some alarm. The Trader agreed to accompany them and once again it produced the unfolding doorway from its chest.

The planet was in its Autumn season, with the fields covered in leaves and a constant cold wind blowing across the landscape. The Trader, Damasin, Nyssa and Tegan emerged from the mirror doorway into a forest where a trail of broken earth and smashed branches led to Damasin’s crashed spaceship. Tegan advised her friends to keep a close eye on Damasin, but Nyssa didn’t think they’d be at risk now he knew they weren’t his assigned targets. The ship was now just a mangled piece of metal with an open hatchway. Damasin entered and then emerged moments later to say everything had been looted, including equipment for which the thieves would have had no possible use. Damasin held up the Doctor’s misplaced glasses and the Trader said that with these he could follow the Doctor’s death signature.

The creature led the way, but for the entire journey they didn’t see a single person left on the war-ravaged planet. Nyssa asked Damasin why he became an assassin and he told her it was something he was exceptionally good at. This made Nyssa angry as her father was killed by someone who loved destruction and chaos, and it distressed her to think that man was still out there, causing pain to others while wearing her father’s face. Damasin pointed out that she‘d feel much better if that person was dead and he suggested she seek vengeance. Nyssa told Damasin he was cruel and reminded him that he too would die one day, but he just smiled and said he could never die. He explained that his cabinet possessed certain qualities that kept him simultaneously within the realm of the living and the dead. It’s what made him the perfect assassin. Nyssa recalled that the Dar Trader had spoken of those who’d experienced the threshold and come back changed, and the Doctor had mentioned dimensional anomalies where two Universes existed as one. She pointed out that Damasin was risking his safety outside the cabinet, but he said he remained in contact with it at all times via a grafted link on his neck, and he’s never met an opponent good enough to make him worry about severance. Nyssa thought this over-confidence made him vulnerable.

The group arrived at the scene of a battlefield and they all stood in shock at the sight of all the limp, twisted bodies lying on the blood-soaked grass. War machines stood nearby in silence, having completed their task. The Trader began picking through the dead, looking for things to salvage. Tegan objected, but Nyssa knew it was a necessary part of their philosophy and they were no worse than the birds that were picking away at the flesh of the dead. One of the bodies was still alive and it attacked the Trader with a knife, but the creature knocked it aside. The attacker was revealed to be a woman, smeared with dirt and wearing torn rags. She was bleeding heavily from a deep cut to her chest and she appeared to have a broken leg. Nyssa calmed her down and assured her they weren’t her enemies, and the woman managed to explain that they were ambushed by military forces and everyone else was killed. Nyssa asked about the Doctor and the woman remembered seeing the military take away a blue box. Suddenly the terrified woman launched into an attack and although Damasin immediately slit her throat, Nyssa found she’d been stabbed by the woman’s knife. As Nyssa fell to the ground, the Dar Trader announced that it must trade with her…

Tegan tried to stop the Trader, but Damasin said it might be Nyssa’s only chance of survival. Nyssa was afraid of going to the same place that consumed her father, but she knew her body was going into shock and that she might die soon. She agreed to the trade, and then her heart stopped beating. For three minutes she was dead, then she woke up and found herself cradled in Tegan’s arms and lying on a carpet of leaves. Nyssa asked if she’d traded and Tegan confirmed that she had. She’d died and the Trader had brought her back to life. She didn’t feel any different and assumed it must have something to do with her Trakenite background, but it made her wonder why her father hadn’t been able to resist the pull of the Master and turn him to stone. Damasin and the Trader had gone to scout ahead to see if they could find the Doctor, and when they returned Damasin was pleased to see Nyssa looking better. The Trader had scented the Doctor a mile away in the city and said they should be able to reach it before nightfall.

When they reached the city they found the militia had smashed all resistance, leaving bodies in the street where they fell. There could hear small pockets of fighting not far away and could see smoke and flames where buildings had been set alight. The few people they encountered turned and ran at the sight of the Dar Trader. They entered a courtyard and found the TARDIS, but the Doctor was on his knees with a pistol held to his head by a rabble of soldiers. He was bargaining for his life, trying to convince them he didn’t work for the militia and that there were no weapons inside his ship. Tegan called out the Doctor’s name and the soldiers all turned their weapons on the new arrivals. Damasin stepped forward and warned the rabble they were about to make a calamitous mistake. He explained that the blue box didn’t belong to the Doctor, but to him. The head soldier refused to listen and gave orders to his men to open fire. Bullets pumped into Damasin’s body, but although he reeled slightly, he remained standing. He appeared completely unharmed, despite blood pouring through holes in his suit. Then he pulled out his dagger and within seconds, all of the soldiers were dead. Damasin’s face betrayed his glee and the Doctor turned on him angrily.

The Doctor greeted Tegan and Nyssa and asked where Adric was. After learning about their experiences on the Dar Trader ship, he explained that he was unable to join them as promised because the TARDIS was pulled to this planet deliberately by a tracking beam keyed to its specific pattern. Fortunately he discovered the tracker in the wreckage of a spaceship and it’s now been disabled. Then he was found by the militia and brought here by a very stubborn General. He’d managed to stay alive by claiming his ship contained superior weaponry, but the plan blew up in his face when the resistance captured him and demanded he hand the weaponry over to them instead. The Doctor was angry with Damasin for injuring Adric, but Nyssa pointed out that he also helped them track him down. The Doctor suspected Damasin may have been responsible for bringing him here in the first place and he produced a metal device from his pocket and threw it back to its owner. Damasin confessed and said he needed a Time Lord to help him power his remarkable cabinet. He’d used many things over the years to straddle the dimensions, but he’d learnt that the energy of a Time Lord sustained it for so much longer.

Nyssa was sure there must be some good in Damasin, but the assassin pointed out that his business was in taking life as well as ensuring the continuance of his own. He unfolded the metal device the Doctor had thrown him and it became a gun. Nyssa placed herself between them, but Damasin assured her she was wasting her time. He had no heart or conscious that she could appeal to and he said he wouldn’t hesitate in shooting her if she stood between him and his quarry. The Doctor moved her aside and Nyssa recognised the look on his face - it was the one he had whenever he was preparing to sacrifice himself. Damasin smiled and fired. White strands of light shot out from the gun and enveloped the Doctor like tendrils. He clutched at his hearts, then the white light seemed to suck something from him and returned to the gun.

Damasin folded his gun away while Tegan rushed to help the Doctor. Nyssa asked the Dar Trader to trade with the Doctor, but the creature said the Time Lord had died before and he didn’t need their help. It asked Damasin to remove his cabinet from their ship, then it unfolded the doorway again. Damasin congratulated Nyssa on convincing him that some people can claim to have perfect innocence, then he and the Trader stepped into the doorway and it folded up behind them. Nyssa and Tegan dragged the Doctor into the TARDIS, but his deathly state affected the ship, leaving it darkened and cold. His companions felt helpless, but they did all they could to check on his condition and make him comfortable. After a while, the lighting inside the ship returned to normal and the Doctor expelled a long breath and then shot up from the floor. He warned his companions about the dimensional anomaly and said they mustn’t let the cabinet use his energy. He said the Traders had made it unstable and the Dwarf Star Alloy could rupture, pouring death out and killing everyone on this planet. He programmed the controls and the TARDIS dematerialised.

They returned to the Trader ship and found the chamber now filled with light coming from the assassin’s cabinet which was now throbbing with power. Damasin greeted them, knowing they’d had to return for Adric. He taunted the Doctor and revealed that Adric was being held at spear point by the Trader. The Doctor warned him he was about to blow a hole in the Universe, but Damasin believed he was bluffing. He claimed he was repairing the damage to his cabinet, but the Doctor told him it had already reacted to the Traders’ tinkering and if he plugged the Time Lord energy into the breach it would simply open it wider. The Trader confirmed that it had planned to join the others and experience the threshold, but it had now been convinced by the Doctor’s words and it turned its spear away from Adric and pointed it towards Damasin instead. Damasin pulled out his own dagger and attempted to slash the Trader, but to no avail. Eventually he plunged his dagger straight into the centre of the Trader’s chest plate.

During the distraction, Adric approached the cabinet, but Damasin spotted him and attacked him with the dagger. Nyssa crashed into the assassin and knocked him off balance, then she reached up and ripped the graft from his neck. The Doctor yelled to her to throw it into the cabinet, but Damasin grabbed her arm and both of them fell into the cabinet and the door folded up and shut behind them. The machine powered up and there was a blast of light, followed by a terrible crack.

Nyssa found herself on the threshold, the place between both realms. It was absolutely silent and when she opened her eyes she saw that she was in a grove that reminded her of Traken. It could easily have been her home and she now realised why the Traders sought it so much. She saw Damasin standing nearby and noted that this was not the first time paradise was destroyed by a creature like him. He demanded she hand over his graft, pointing out that he couldn’t use the Doctor’s power without it, but she refused. He threatened to send her to her father, but this just made her angrier. She decided that she did want vengeance after all, so she took the graft and placed it on her own neck. There was another crack and the grove began to break up around them. A great wind blew up sending leaves into the air and causing great stones to come crashing to the ground. Damasin noticed that his feet had turned grey, then slowly his suit turned into stone. The effect raced upwards to cover his legs, then his torso. He was becoming calcified and Nyssa realised she was the one causing it, taking the Doctor’s energy into herself and bonding herself with the machine to deny the assassin his very existence. Damasin pleaded with her, but then his face turned to stone too and when the transformation was complete he was nothing more than a lifeless grey statue. The grove continued to break up around her and she remembered the Traders telling her that death was attracted to her. She stood there, waiting to be erased, but then she heard a whisper which slowly became louder. It was the Doctor urging her to wake up…

Nyssa did wake up and found herself back in the TARDIS. She was in her own room, lying in her bed. She asked what happened and he told her the cabinet didn’t rupture as she’d managed to switch it off herself. The Doctor told her it was the Dar Trader that brought her back to reality, although he’s not sure how the creature did it. Nyssa asked about the stolen energy taken from the Doctor, but this made him uncomfortable. He was sure it would have rejected Damasin, but he’s not so sure it would have done the same for Nyssa. He told her to get some sleep and for the first time in days, she didn’t dream.