Susan was convinced there was no way they’d be able to survive the impact of the approaching missiles, but then suddenly space itself seemed to rip open from a point near the Nevermore to a point infinitely far away. There was a flash of impossible light, made up of colours nobody had ever seen before, and then something squeezed through the gap. They saw tentacles made up of glowing energy which wrapped themselves around the alien spaceship and dragged it back into the fissure. Smaller tentacles then did the same with each of the missiles.
Everyone looked at the screen incredulously and wondered what had happened. The Doctor explained that this was one of the monstrous beings that lived beneath space, always hungry and always alert. Thanks to the Benchmarking, it’d been able to break through into normal space. They picked up a distress message coming from the other vessel and listened as the other captain warned that his ship had been destroyed while engaging a belligerent enemy vessel that had been sabotaging space around their major trade routes. The alien captain believed they’d been attacked by the Nevermore and accused them of a deliberate act of aggression. He warned any nearby forces that their space was being invaded, then he screamed…
Susan realised the captain had simply been protecting his borders from what he thought was an act of aggression. She accepted that it shouldn’t have opened fire on them, but then perhaps the Nevermore shouldn’t be going around opening up holes in space near other people’s star systems. The Doctor turned on Rostrum again and told him his actions were not only dangerous to the fabric of the Universe, they were also inflammatory to other races. Rostrum said they’d assumed other people would be grateful as they too would be able to use the process to aid their own navigation. The Doctor said space here had just frayed apart and it probably ran all the way to the last Benchmark. Ian wondered whether this area could be cordoned off, but the Doctor said any small disturbance might cause another rip between two holes and the problem could escalate with more and more rips forming until a whole section of space detached and floated away, leaving the creatures behind to feed at their leisure.
The Doctor told Susan there was a possible solution - he might be able to use the immense power from the TARDIS’s time engines to heal the fissure, but he was worried it would destroy the TARDIS. They’d be left here without a means of transport, stuck in one time and place forever. Susan argued that at least they’d have saved the Universe. The Doctor returned to Captain Rostrum and they started arguing again, and when Ian and Barbara stepped in to act as peace makers, Susan decided to find the crewman she met earlier to see whether the engines could be repaired.
Susan found herself in an area of the ship she’d never seen before. Large power cables fed into spherical tanks the size of a small asteroid. Once again, Rostrum’s leaves and vines were dying and Susan began to wonder just how widespread the problem was. The crewman appeared and told her she had a habit of wandering into areas she shouldn’t be. He told her this was the place where the gravitational singularities that trigger the Benchmarking were stored. Each of the seven tanks contained one singularity, trapped inside a magnetic Klein bottle. By dropping the magnetic field briefly, one of the singularities would momentarily plunge through the skin of things like a needle, then the singularity would be pulled back when the field was re-established. Meanwhile, the other six singularities would inscribe gravitational information on the boundaries of the hole, so that any traveller who used the Benchmarks would know where they were.
The crewman told her his position here was only temporary as he liked to move around a lot, clearing up messes and getting involved in dangerous situations. Susan thought he sounded very much like her group and asked if he had his own ship. His answer was a bit vague so she told him about the Doctor’s plan, adding that they were risking their own ship. The crewman said Captain Rostrum could take them somewhere safe and Susan thought the Doctor might even be able to build another TARDIS eventually. Susan said she’d go mad if she had to stay in one place as she wanted to experience the Universe, not just observe it from a distance. The crewman told her it would be impossible for her to go with him, but he could probably drop her off somewhere. She asked if he ever got lonely, but he said there’s a difference between being alone and being lonely. He said he was sure she could look after herself and that she didn’t need a grandfather figure to fall back on, but she was scared by the prospect of heading off on her own.
Susan started to look unwell again, so he suggested she return to her friends. Before she could do so, she cried out and collapsed. She pleaded with the crewman to help her, but he said he couldn’t and walked away! Susan couldn’t believe he’d left her and she tried to call after him, but she couldn’t make a sound. She wondered if she was suffering from the same thing that was affecting the vines and the leaves? She drifted in and out of consciousness and was only woken up when Barbara came rushing over after getting worried that she’d been gone so long. Barbara helped her to her feet, but when Susan asked her what happened to the crewman, she said she hadn‘t seen anyone.
By the time they returned to the bridge, Susan was almost back to normal. The Doctor was still intent on staying behind and using the TARDIS to repair the fissure, and she just hoped that their sacrifice wasn’t going to be in vain and that another Benchmarking ship wouldn’t come along soon and cause the whole problem again. Ian was struggling to understand the concepts and kept coming up with analogies that just confused him even more. Barbara turned furiously to Rostrum and told him off for letting one of his crewmen walk off and leave Susan unconscious on the floor. The Captain assured her he had no crew and that he was alone on this ship. Susan said she’d talked to the man twice, but when Ian asked her what his name was, she realised she didn’t know.
The Doctor guessed they had another intruder aboard, but Rostrum suspected he was one of their group, engaged in sabotage while they were distracting him. The Doctor pointed out that Rostrum had eyes everywhere, but Barbara reminded him some of the leaves and branches were dying. Ian asked Susan whether the leaves were alive or dead in the areas where she met the man and she realised they were dead in both cases. Ian thought the man might be taking advantage of those areas to creep around, but the Doctor suggested he might have deliberately caused the dead areas himself to cover his tracks! He turned to Rostrum and asked for maps of all the areas where the plant no longer had any senses. Rostrum was suspicious, but the Doctor pointed out that the stranger was undoubtedly hiding in those areas. The group decided to split up and each of them would search the areas one by one. Barbara suggested Susan stay behind as she was still unwell, but Susan refused to be treated like a child any more. The Doctor looked at her curiously, as if he’d suddenly realised she was growing up, and it didn‘t seem like he was entirely comfortable with the idea.
Susan already knew where she’d seen the mysterious crewman before, so she knew roughly which areas of the ship he’d been working on. Spreading out from there, it wasn’t long before she found him, standing in the control room, not far from where the TARDIS had originally landed. He was pulling the lever that operated the Benchmarking process and she noticed the nearby tendrils had all turned brown and twisted. Susan realised it was him who was killing the plant and that everywhere he went, the leaves and vines were literally dying. He argued that the plant was killing so many others through its actions. He told Susan they were opposites of each other and he warned her not to get too close. Susan became weak again and had to sit down. He revealed that he was draining her life away just as he was doing the same to the plant. It wasn’t through choice and he didn’t want to be responsible for her death. She realised he wasn’t human and told him she’d keep following him until he told her what he was doing here, even if it killed her.
Ian and the Doctor suddenly leapt out from a side corridor and attacked the man, pulling him down to the ground. They quickly dragged him to the storage area where they could restrain him, but both Ian and the Doctor were starting to look pale so they had to take turns while the other rested. Simply standing at a distance for a couple of minutes seemed to be enough for them to recover their energy and after a while, Barbara joined them and helped out. As they moved down several corridors, the leaves and vines they passed started to wither and die, then they gradually recovered again after they‘d moved on. Eventually the intruder was tied up and locked safely away. It only now occurred to Susan that the man and the Doctor both looked very much alike. They were both old and white haired and they were both quite imperious in their looks and attitude. Yet, there was something softer about the intruder that she rarely saw in her grandfather. The Doctor suspected the man had some sort of connection with the fissure outside the ship and wondered if he knew a way of fixing it.
The Doctor and Susan decided to question him and they stood on either side of the room to avoid getting too close. The man confirmed that he was from the other side of the fissure and said they had to free him or else the most terrible things would happen. The Doctor believed only monsters lurked beneath the surface of space, but the man laughed and said that’s what his people said about this side. Of course, in his case, his people believe they’re living on the surface and this space is underneath. The Doctor was surprised to discover there were civilisations there, including intelligent ethical life, but the man says it didn’t really matter which side of the divide you lived on. His race were formed of different patterns of energy and different structures of matter, which is why when he took on human form, he based it on an image he picked up from Susan’s mind. He added that because they’re opposites, his physical presence was draining their life, and if they were to cross over to his realm, they’d do the same to his people.
The man revealed that the actions of this ship had been causing great distress to his people. Vast tracts of space have been lain waste by the effects of the Benchmarking, so he came here to stop it. He was prepared to use force if necessary, but he’d prefer to use guile or even just intelligent conversation. The Doctor announced that they’d help him and told Susan to release him so they could escort him to Captain Rostrum.
Suddenly they heard Ian calling for help, saying that he and Barbara were held fast in Rostrum’s vines. The voice of Rostrum boomed out, claiming they’d betrayed him and that he knew they were all working together to sabotage his ship. He insisted the Benchmarking would continue and said they’d stay here as his prisoners and rot for as long as it took him to complete his mission. Ian pleaded with the Captain to think logically and pointed out that their presence had no effect on the plants so they couldn’t be working with the intruder. Unfortunately the Captain had never seen or felt the presence of the intruder and he only had their word that the man even existed. The Doctor reminded the plant about the catastrophic dangers inherent in the process, not only in the Universe but the other one too, but Rostrum refused to believe his words.
The Doctor asked the intruder for his assistance and the man said he knew enough about the way Benchmarking worked to seal the fissure, but he needed to return to his original form in order to do it. Susan urged him to do it, but if he changed form right now he’d kill every living thing within half a light year. Susan urged him to do it anyway and not to worry about them, but the Doctor said the TARDIS sat outside time and space and they’d be safe inside there if they could make it. Unfortunately Ian and Barbara were captives of Captain Rostrum and he’d never let them go. Susan asked the intruder if he could change his form slowly enough to weaken Rostrum’s binds and allow them to escape before they were harmed. He wasn’t sure if he could hold it back so long, but he agreed to try.
The man’s body began to glow and shift. Susan was worried that healing the fissure would require all of his power and prove fatal to him, but he told her to leave immediately. The Doctor and Susan ran from the area as the man’s body transformed into light, darkness and chaos. Ian managed to pull Barbara free from the vines and then together they all staggered back to the TARDIS. They were on their hands and knees when they arrived and the Doctor dropped the key. He was too weak to pick it up, but Ian found it and they managed to get inside.
They stayed in the TARDIS for hours, recovering their strength. When they eventually emerged, they found Captain Rostrum was dead, but on the viewscreen there was no sign of the fissure. The man had sealed it with his life. Ian wondered what would happen to the Benchmarking ships and the Doctor said he hoped humanity would learn its lesson, but he doubted they would. Susan noticed a flashing light on the control panel and thought Rostrum had recorded a final message before he died, but when they played it back, it turned out to belong to the alien man. He was calling out an emergency to all civilian and military forces. He said he’d been fatally injured while repairing damage done during an innocent encounter with a vessel from the other side of the spatial membrane. He said the vessel had accidentally weakened the fabric of space around their major trade routes, but when he confronted the occupants they showed great courage in assisting with halting the damage. He said they weren’t the monsters they’d always believed, but all further contact should be avoided as their essential natures couldn’t co-exist.
Susan knows that she'll always remember the man. She’s not sure whether he left some small part of himself behind, lingering in our reality, or whether she just wished he had - but she’s sure she can still feel him. It’s what finally gave her the courage to leave her grandfather and make a life for herself. She knows that while she might one day be alone, she’ll never again be lonely.