The wind howls across the surface of Phobos, the largest of the two moons of Mars, as the TARDIS materialises in the year 2589. Lucie emerges, but she's not impressed by their surroundings. They appear to be halfway up a mountain (again), and she wonders whether the Doctor has a licence to pilot the ship or just makes it up as he goes along. Before he can join her, a large net whistles through the air, enveloping Lucie. The Doctor emerges and reminds her of the dangers of leaving the ship before checking what's outside, although she claims that he's never told her that before. Just then, they're joined by two young men who are relieved to find that the "animal" trapped inside the net is just a young girl. Together with the Doctor, they help to cut Lucie down.

Elsewhere, an inner airlock slides open and two new arrivals disembark from the economy space shuttle. One of them, Farl, is a huge beast of a man who complains constantly about the cramped seating -- which was obviously designed for the smaller human frame -- and the fact that the steering on their ant-shaped buggy doesn't seem to be working properly. His partner, Amy, tells him it's safe to remove his helmet and asks whether he regrets coming to Phobos, but he thinks her idea was a good one as no one will come looking for them here. They're greeted by an older woman, Eris, who noticed that they were having problems with the buggy and offers to look at it for them. She explains that she's the camp's engineer and although her position here isn't official, no one else knows how to repair things. In fact, nobody is actually in charge of the Lunar Park and it's been left to look after itself. There was going to be a hotel with a botanical garden once, but the construction had problems and the owners only got round to finishing the environment dome. For years the company said they were going to build the rest, but they never did, so Eris's people claimed squatters' rights. Farl finds the air too cold, but Amy admires the way the ice is set into the rock, like veins of diamonds. Eris says that this is quite rare, and it was a big part of their argument against building a resort here. Some of the most spectacular valleys on Phobos are located right underneath the dome and her people believe they should be left as they are. Amy promises she and Farl will do their best not to spoil things. Eris is unable to fix the buggy at once but promises to look over it again later.

Lucie rebukes the two men, Drew and Hayd, for setting up the animal trap and suggests this is probably the only way they can pick up a girl. The men say they were both worried about a monster in the area; just before the Doctor and Lucie arrived they heard a weird noise, like a wheezing and groaning. The Doctor points to his TARDIS and tells them his craft makes that noise when it lands. The young men tell them that another man from the camp claims to have seen a monster, although no one really believes him. They're currently inside the Martian Lunar Park, an environment dome on Phobos, which answers the Doctor's questions about why the gravity feels "wrong". He demonstrates by jumping around like an excited child, which embarrasses Lucie. The Doctor launches into a lengthy scientific explanation about how the moon's unusual ellipsoid shape and its proximity to Mars means that gravity varies massively across the surface of Phobos. The settlement obviously compensates with an opposing field across the top of the dome that maintains the atmosphere. Lucie yawns rudely and the Doctor apologises, saying he's known savages with better manners. Drew and Hayd tell him they've come to this mountain because of the wormhole on the top. It's not a real wormhole, of course, but they call it that because it's a natural gravitational anomaly which forms a vertical tunnel that people can jump down without ever hitting the sides. The Doctor doesn't think that sounds particularly natural, and Lucie guesses this means he's going to want to see it for himself. He asks the young men what they're going to do when they get there, and they tell him they're going to jump down it...

Amy and Farl settle into the camp and decide to set up their portable cabin -- but Farl makes an error when he pulls the cord and it instantly 'inflates' the wrong way round. An older man, Kai Tobias, spots that they're having a bit of trouble and offers to help. They introduce each other and then Kai and Farl struggle to turn the cabin the right way up. The man asks whether they're with the Drennies, the nickname adopted by 'adrenalin-junkies' who get off on extreme sports, but Farl explains that they're just travellers. The settlement doesn't get many of those coming here these days, and Kai asks why they chose Phobos, but Amy simply replies that they needed to get away from it all. Kai advises them to take care of themselves, as this can be a dangerous place, but Amy assures him that they're not planning anything like rock-climbing or white water rafting. Kai didn't mean that -- he was warning them about the monsters who come from the wormhole up on the mountain.

The Doctor, Lucie, Hayd and Drew eventually make it to the top of the mountain and stop for a breather. Lucie is disappointed as she was expecting to see something more than just a hole in the ground. The Doctor cracks a weak pun, and when Lucie threatens to punch him on the nose, the young men tell her she'll be okay, as there are no laws on Phobos; the place was set up years ago by a group of hippies who weren't very big on rules. The two men discuss which of them is going to go first and Lucie realises they're planning to bungee jump down into the hole. The Doctor shouts down and discovers there's no echo, which is more evidence that the anomaly isn't natural. The men claim that everything on Phobos is natural and they come here because it's impossible to get a properly authentic experience on Earth any more. No one knows how far down the hole goes as no one's ever reached the bottom. Whoever chooses the longest rope and goes the furthest down into the hole gets the most respect from among the Drennies.

Amy asks Kai whether the monsters are real or whether he imagined them, but he assures her he's seen them for himself. He asks how much she knows about this place and she says she read about it in a brochure before coming here. As Mars was named after the god of war, its moons were named after his sons, Deimos and Phobos. Kai points out that Phobos comes from the Greek word for fear, and he says the people who named it had no idea how right they were! He tells her this is a treacherous place, which is why the Drennies come here. He calls the creatures Phobians and says they look like "strange animal things". He says you can tell when they're nearby as they make a terrible wrenching, screeching noise. Amy is surprised people still come here, as there were dozens of holidaymakers on the shuttle today, and Kai admits that though he's been trying to warn people for years that the Phobians will one day attack them, they never have, so now everyone assumes he just made the stories up for fun. They hear a strange noise, and Kai fears it might be one of the monsters, as they're getting closer every day...

The rope being used by Hayd and Drew will stretch to around 3,000 metres and will snap back very quickly when it reaches its maximum length. Everything seems secure at the top of the mountain, so without further delay, Hayd races forward and leaps into the hole. Lucie thinks they're insane, but Drew says it's the best feeling anyone can have, although he admits that he never goes first as he still gets nervous. This is the reason people come to Phobos, as well as other activities like grav-board runs outside the dome, or swimming in the melted ice floes underneath the surface; it's even great for orbit-hopping if you've got the equipment. A signal indicates that the rope has reached maximum extension, so Hayd should be on his way back up any moment. While they're waiting, Lucie asks to borrow the Doctor's binoculars, but when he points out that it's too dark in the hole to see anything, she says she doesn't want them for that. He passes her a telescope and she looks at a pool in the distance. She's spotted a girl just sitting there, with a man laid out on the ground, not moving. She wonders if they're OK and while the Doctor takes a look, Hayd emerges from the hole. The Doctor asks if they can pass by the pool on the way back to their camp so they can check on the couple. Lucie is concerned about climbing all the way back down the mountain, but the two men have already programmed a buggy to come and pick them up.

Amy asks Farl what he thinks they should do, but Kai recommends they leave as soon as they can get another shuttle. Eris joins them and rebukes Kai for telling them such claptrap, hinting that he usually saves such stories for the Drennies as they find them exciting. She asks him to go and check on Rosa as the poor girl still isn't awake. Kai says goodbye to the visitors, and once he's gone, Eris apologises for his behaviour. She assures Amy and Farl that there's no such thing as Phobians and says Kai has been saying things like this for years. Amy swears she heard one a few minutes earlier, but Eris is sure it must have been something else. In all this time, nobody except Kai has ever seen one of the monsters and his stories aren't even consistent. He keeps changing what they look like and what they do to people. Eris doesn't mind him teasing the Drennies, but she'd prefer that he be more honest with her. She and Kai aren't married, but they've been together since the old days. She always told herself she'd stick with him whatever, but she keeps thinking something terrible is going to happen because of his stupid ghost stories.

The Doctor, Lucie, Hayd and Drew approach the area where they spotted the girl and the apparently injured man. The two Drennies recognise the girl as Chrissie, who they met earlier at the camp; it looks like she's been down in the ice again. They stop the buggy and the Doctor tries to ask Chrissie what happened to her friend -- who the men identify as her boyfriend Scott -- but she's shaking too much to respond. The Doctor sees that the regulator in her wetsuit is still working, so she's not suffering from hypothermia but is in a state of shock.

Eris returns to the cabin she shares with Kai, and they agree that Amy and Farl seem like nice people, although Kai is disappointed they didn't believe his story. Eris understands why he doesn't like the Drennies, but she asks him to leave the new arrivals alone. He says he's only trying to help everybody, but she warns him that if he keeps doing it, she'll leave, and then what would become of him? The door bursts open and Drew calls for them to come outside where the Doctor, Hayd and Lucie are examining the body of Chrissie's boyfriend. They've never seen anything like it before and they never want to again. Eris and Kai join them and Lucie introduces herself and the Doctor as travellers. Chrissie has been taken to the med-cabin and is under sedation, but fortunately she seems all right physically, although she's exhausted after fighting her way out of the ice caves. Kai is convinced Scott was killed by the Phobians, and the Doctor confirms that something ripped him almost in half. There seems to have been no purpose to it as he hasn't been eaten, so it was a senseless, vicious attack, and not something a human could have done. Kai feels vindicated, but Eris just wants everyone to get inside as it'll be dark soon.

The next morning, Drew is woken from a nightmare by Hayd, who is shocked by his friend's nervous reaction. Drew explains that he thought Hayd was one of the monsters that tore Scott to bits and admits that he hardly got any sleep last night as he heard something stalking around outside the cabins. Hayd assures him that even if the creatures were here, they didn't do anything. He reminds Drew that they'd agreed to go grav-boarding outside the dome today. Drew wants to cancel their arrangements because of what's happened, but Hayd thinks Scott probably antagonised the creature and they should be all right as they can handle themselves. Reluctantly, Drew agrees to go with him.

The Doctor finds Lucie in the main cabin drinking her third cup of coffee. She didn't get much sleep last night either and can't get Chrissie's face out of her mind. They overhear part of a conversation between Amy and Farl; she fears that they'll be found out sooner or later, but he assures her they'll be all right if they keep their mouths shut. After they've gone, the Doctor and Lucie agree that they make an odd couple and they seem different to everyone else here. Eris overhears and confirms that Amy and Farl are indeed different. Lunar Park used to be a place for people who want to escape from the pressures of modern life, but people like that don't really fit in any more. When the Drennies started coming here, word spread and now they've more or less taken over. This is a dream place for fans of extreme sport; it's 100% natural and has the perfect environment for all sorts of activities. The Doctor is curious to know how she can be sure it's natural, and she tells him it's all been checked out by archaeologists and the terrain was charted even before anybody thought of building a dome here. Lucie is hungry, so Eris tells her she can get food at the mess tent; they only need to pay what they can afford. The Doctor wonders how they can keep the place going with such a laudable policy, and Erin says that a lot of it runs itself and they only have to hire out equipment and buggies. The Doctor asks about weapons to use against the monsters, and Eris becomes uncomfortable, so he decides to talk to Kai to find out more about what they're up against.

In the food tent, Amy and Farl are arguing. She tells him she doesn't think they should go ahead with their plan, so he storms out, telling her she can find him once she's changed her mind. Lucie arrives and sits next to Amy, noticing straight away that she seems upset. Amy wants to leave, but there are no shuttles due for another three days. As they discuss the Drennies, Lucie makes a reference to how daft things seem in the future and when Amy looks confused, she admits that she's a time traveller from the past. She asks Amy what she and Farl are doing here with all the thrill-seekers, but the young woman doesn't answer and invites Lucie out for a walk instead. Elsewhere on the camp, Kai finds a depressed Farl chucking stones on the water. The old man knows tensions are running high here, so he wanted to come over and apologise for the unpleasant remarks people have been making about Farl's race, the Githeans. Farl says he hadn't heard them, but Kai tells him that he'd assumed that was why Farl stormed off. Eris and the Doctor arrive and ask to talk to Kai privately. As they head for his cabin, Eris tells him the Doctor has offered to help a woman called Rosa who took a nasty blow to the head and has been unconscious since yesterday morning. Farl calls out to Kai to find out what people have been saying about him, but it's too late.

Drew and Hayd are preparing for their grav-boarding session outside the dome, but Drew is nervous as he hasn't done this before and he asks his friend not to go too fast. Hayd tells him to relax as it's not too different to snow-boarding. The rocks all down the gully are angled, so if they hit each one dead on, the anti-grav boosters on their boards will cushion the impact. The board will then make them rebound in the low gravity to give an effect similar to bouncing off a snow ridge, only way cooler. The best part is the apex where they can try spins or flips. They're ready to go and Hayd leaves first, with Drew following behind him a few seconds later.

Eris apologises to Kai for not believing his story about the monsters; everyone thought he'd imagined it because no one else had ever seen them. The Doctor asks Kai to tell him everything he knows, and the old man says the monsters have always been here. He thinks they're what stopped the building of Lunar Park in the first place. He saw them attack one of the kids once, and although he survived, he refused to tell anyone about it in case people thought he was mad. Sometimes Kai even hoped there'd be another attack so that people would start believing him, but Eris reveals that some of the Drennies even got excited by the stories, like daring each other to stay in a haunted house. Even after the latest attack, not everyone believes it, and Kai has heard people talk about the dead body being a fake and the whole thing being a publicity stunt. The Doctor wonders why Kai hasn't left already if they're all in such danger, but the old man points out that the Doctor hasn't left either...

Amy and Lucie climb into their spacesuits and Lucie starts complaining that they weren't designed to flatter her figure. As they wait for the outer airlock door to open, Lucie asks about Amy's relationship with Farl and learns that the big hairy man is a Githean; unfortunately their race are forbidden to marry outside their own species as they've had problems maintaining the purity of their gene pool. The two girls move out onto the surface of the moon and Lucie is overcome with awe at the sight of Mars just above them. Amy explains that Farl's family are not particularly friendly and even hired a hunter to bring him back. Amy herself is considered 'expendable', which is why they're hiding out here until Farl's family loses the scent. Farl has decided he and Amy should split up for her own good, but unfortunately there's an even bigger problem -- Amy is pregnant with Farl's child, although he doesn't know it yet. Suddenly their spacesuit intercoms pick up a signal from nearby and they hear someone screaming. It can't be coming from far away as the headsets only have a limited range, so they both race off to see what's going on.

The Doctor asks Kai what the creatures look like and he says they're tall, maybe three or four times the height of a man. They're shapeless, brownish-grey and with lots of arms, but they have no face and make an abrasive screeching noise. He's interrupted by a signal from the emergency communicator. Eris answers it and hears Lucie calling from the airlock. She tells them they've found another body, this time outside the dome. Amy has identified the victim as Kelly, a man she met on the shuttle to Phobos. The Doctor is angry with Lucie for going for a walk outside, but Lucie hasn't got time to discuss the matter and tells him to come quickly. Eris offers to go with the Doctor, so they ask Kai to stay behind in case Lucie contacts them again. There might still be Phobians in the area, so the Doctor wants to collect together anything that might be useful against them. When Eris says they don't really have anything, he tells her they're going to have to be creative.

Drew is exhausted when he reaches the bottom of the gully, but he can still hear Hayd's ecstatic cries on the intercom. Hayd has decided to move on and is now grav-boarding down the next ridge. Suddenly Drew spots something huge moving behind one of the rocks about twenty metres in front of where Hayd is heading. He calls out a warning to his friend, but Hayd thinks he's playing a practical joke and ignores it. Then Drew hears Hayd screaming and the radio system breaks down...

Amy and Lucie have witnessed the attack on Hayd and decide to investigate, confident that the Doctor will be along soon to look after Kelly's body. They meet up with Drew, who tells them a monster reached out and knocked Hayd off his board. His friend is badly hurt and needs their help, but fortunately the board knocked the creature off balance too and it fell down one of the craters. Lucie decides to take a look to see if it's dead, but as she leans over the edge, the monster lashes out at her. It's hanging from the cliff edge, but it hasn't found its footing yet and is struggling to climb back up. Lucie starts lobbing rocks at it, hoping that it will come after her and she can lead it away from the others. Unfortunately she just makes things worse and the monster starts screeching...

The airlock opens and the Doctor and Eris emerge onto the surface of Phobos. They've emptied the airlock's emergency kit and they have ropes and grappling hooks, and there are medical supplies aboard the buggy. As they move off, the Doctor realises how odd it is that the moon has an atmosphere, albeit a very thin one. The Doctor contacts Lucie on her intercom and learns that she's trying to lure the Phobian away from the area while the others look after Hayd. What she's doing is absurdly dangerous and her only hope seems to be that the monster might accidentally fall into a crevasse. The Doctor tells her that he and Eris should be there soon, but in the meantime he's detected a large rock not far away and he advises her to climb up onto it. Lucie reaches the top of the rock and can see the buggy approaching. The Doctor throws her a climbing rope and tells her to lasso the Phobian. The other end is attached to the buggy, and when Eris turns the engine to full, they're able to drag the creature away. Eventually it stops moving and Eris thinks the creature must be dead or unconscious. The group reunites with the others and then the Doctor examines the Phobian more closely. He discovers the creature is made of metal and is covered in some form of sackcloth, and Eris identifies it as a construction droid, a robot that was left behind by whoever built this place. The arms are actually tools, including one that's designed for drilling. The robots are usually pre-programmed to function autonomously, doing routine tasks such as terrain mapping, surface repairs or mineral extraction. Eris supposes this one simply malfunctioned, but the Doctor thinks it's unlikely that would make it attack people on sight. He suggests the droids can be operated remotely, so it's more likely someone is deliberately controlling them. And he thinks Eris knows who's behind it all...

Farl returns to the camp and passes a group of young men who start mocking him for being a Githean. He stops to challenge them and the loudest hothead backs away, refusing to be drawn into an argument. Farl punches him in the face, then turns to the rest of the group. A brawl ensues and eventually the leader of the group, suffering from a broken nose, says he's sorry and begs Farl to leave them alone. Kai arrives and tells a surprised Farl that his wife has returned after having a nasty experience outside the dome. Farl races off to the medi-cabin and Kai sends the injured visitor there too, promising to have him patched up in no time. Kai complains loudly about Farl's kind coming in here and thinking they can push everyone around. They're no better than savages...

The Doctor supervises the arrival of Hayd into the med-cabin and tells a relieved Drew that his friend is just unconscious. They're interrupted by Farl, who bursts in and rebukes Amy for going outside where it isn't safe, but she points out it isn't particularly safe inside the dome either. Lucie steps in to defend Amy, but Farl tells her he doesn't trust her and warns her and the Doctor to stay away from them, adding that if they refuse, he'll make them. The Doctor isn't intimidated by his threats, as he's seen things that would freeze Farl's blood. As soon as they're alone Amy rebukes Farl. The young man with the broken nose then arrives and tells Farl that he's been talking to the others and they all want Farl to leave the camp. Amy is shocked to learn that her husband punched the man just because he was insulted. She's no longer sure what Farl is capable of and she runs off, upset.

As they head for Kai's cabin, Lucie asks the Doctor about the terrible things he claims to have seen and is shocked to discover that he's witnessed solar systems being destroyed and whole civilisations being annihilated. They meet Kai, but before the old man can say anything the Doctor warns him he's in a bad mood and doesn't want to hear him say "what are you talking about?" at any time during the conversation. The Doctor then states that the Phobians are re-fashioned construction droids and he accuses Kai of controlling them. Kai says "what are you talking about?" and the Doctor smashes some ornaments in frustration. The old man eventually admits that it's true, and the Doctor guesses he's been trying to drive the Drennies away in order to get the Lunar Park back to the way it originally was. Kai denies this and points out that people have died because of what he's done. In fact, he's been trying to save everyone from something even worse! The Doctor argues that he can't possibly justify what he's done, but Kai was about to ask the Doctor to help him. Realising that would be pointless now, he changes his plan and pulls out a weapon and shoots both the Doctor and Lucie. He then summons one of the droids and orders it to take the two unconscious bodies to the wormhole...

Farl catches up with Amy and tries to apologise, but she's too scared of him to listen. He promises he would never hurt her, but before she can respond, they're approached by Eris, who tells them Drew has just spotted one of the Phobians taking the Doctor and Lucie up to the mountain. She admits that the Doctor thought Kai was controlling them, but now they can't find either Kai or the Doctor anywhere. Eris and Drew are planning to take a buggy up to the mountain but she needs someone to stay behind to look after Hayd. Amy volunteers and asks Farl to go with the others in case they need someone strong.

At the top of the mountain, Kai wakes Lucie up. She soon discovers that she and the Doctor have been tied up and she's unable to do anything more than just struggle. Kai has already informed the Doctor, at great length, about his plans and not surprisingly it involves killing them. She imagines he's going to get one of the fake monsters to rip them to pieces, but in fact he's planning to throw them down the wormhole. Nobody has ever hit the bottom before, so they'll be the first ones -- unless it's bottomless of course, in which case they'll just keep falling and falling. Suddenly a net shoots through the air and knocks Kai unconscious. The Doctor and Lucie are relieved to see Drew, who was dropped off some way down the mountain by Eris and Farl so he could climb the rest of the way without attracting attention. Eris pulls up in the buggy and tells Drew to untie the Doctor while she sees to Lucie. The Doctor assumes Farl has disabled the Phobian robot, but in fact he hasn't and the monster suddenly appears and moves towards them. Although Kai is no longer controlling them, this droid is still obeying his last instructions. The Doctor stands in front of the others and uses his sonic screwdriver to unscrew the robot's leg, and a few seconds later the machine crashes to the ground.

Just then they hear a strange noise coming from the wormhole, and they wake Kai up to find out if he knows what's down there. Kai reminds them that he was protecting everyone from something worse, and tells them that the wormhole is home to a god who comes from another reality that's consumed by fear. Its world was destroyed and the god escaped the collapsing universe by finding a way into our world through the wormhole, which is actually a singularity bridge created by the entity itself. However, the physical rules of the two universes are completely different, so the god has been stuck inside the wormhole. No one knows how long it's been in there as its concept of time is also different. The Doctor realises nothing on this moon is truly natural, and all the sites used for extreme sports have actually been crafted by the entity using whatever influence it could manage. Even the atmosphere was created in order to intensify people's fear by allowing them to hear things like screams. The entity feeds off fear, and if it can get enough of it, it might just be strong enough to break through into our universe. Drew thinks Kai has been helping it to spread terror across the park by frightening people with the droid monsters, and Farl suspects Kai deliberately set him up against the youngsters in the camp. Kai denies this and explains that fear is a powerful thing, but it's difficult to digest, and if the subject doesn't enjoy being afraid then their fear has a poisonous effect on the entity. Euphoria is the spoonful of sugar it needs, in the same way that the classic dictator wants to be feared and loved at the same time. In fact, pure fear actually harms the entity. At first, Kai tried simply warning people about the entity, but nobody believed him so he started telling stories in the hope of either driving the Drennies away or getting some real fear going in order to hurt the entity. When no one believed his stories about the monsters, he was forced to create his own and use them to kill the Drennies. Eris is horrified, but Kai points out that the real problem is that the Doctor is actually feeding the entity himself. He can sense it getting stronger, because the Doctor thrives on saving people and being on the edge, and plays for much higher stakes than the pathetic kids that come here.

Suddenly a booming god-like voice comes from Eris's mouth, confirming that the Doctor isn't happy unless he's risking his life for the benefit of others. All this time, what the entity really needed was him, and it thanks him for setting it free. Kai realises the entity is starting to break through, but he always assumed that if that ever happened it would go through him rather than Eris. The entity learned about this universe from Kai, but it also learned that he was too rational to enjoy fear, so it has chosen instead to speak through the person he loves. It plans to do the same to Farl, speaking through the voice of Amy, the one he loves... and then it will speak to Drew through Hayd. Drew denies being in love with Hayd, but the entity knows better; this is why he always follows his friend around and does everything he does. The entity plans to invest their weak bodies with all its powers and twist the living matter of this universe into a new body for itself. The Doctor challenges it to follow through its threat. So far he hasn't seen the entity do anything but talk, so he dares it to do its worst. When nothing happens, the Doctor calls on Farl and Drew to grab Eris and tie her up; the entity still hasn't fully broken through and Eris is no doubt fighting it from the inside. It has no pseudo-godlike powers yet and all it can do is goad them. For a moment, the real Eris reasserts herself and pleads with the Doctor to hurry as she can't hold on much longer. The Doctor urges the others to attach Drew's bungee equipment to Eris and himself. He reminds Lucie that Kai was hoping to destroy the entity by throwing the Doctor and her into the wormhole, which acts as a magnet to draw fear in. The closer the subject is to the wormhole, the more likely it is that their genuine fear will harm the entity, or at least weaken it. Apart from the fact that the Doctor and Lucie would have died, Kai's plan was quite good. The entity speaking through Eris warns them to stop what they're doing and for the first time they can sense a little bit of fear in its voice. The group is ready, so they haul Eris up and, still attached to the Doctor, they throw her bodily into the wormhole.

The Doctor and Eris fall into the darkness at great speed. The Doctor tells the entity that he does have fears -- things he doesn't enjoy and things he's seen and never wants to see again -- but he finds their current situation quite bracing. He offers to show her his fears and begins with Evil from the Dawn of Time. The entity refuses to be fooled and insists that the things it can see inside his head aren't real. He assures it that he couldn't make these things up and that he genuinely saw them all. The entity demands to know how he could possibly have survived, and he explains that instead of denying or giving into his fears, he faces them. That's what makes him stronger and why he won in the end. The entity pleads with him to stop and the Doctor taunts it, asking if it's fed on too much fear. If the fears he's showing it aren't to its liking, he offers to try some others. Evil from the past is one thing, but he's seen the future too. It scares the living daylights out of him, but what will it do to the entity...? It cries out in agony and the Doctor realises Kai was right -- it can't cope when the fear is real. But the worst is yet to come -- now the Doctor wants to show the entity the things he's afraid he himself might do one day. The entity screams...

By the time they return to the top of the wormhole, Eris has stopped breathing. The group surrounds her body and try to revive her, but it's too late. Lucie doesn't think the entity had any intention of letting Eris go afterwards. The Doctor appears at the edge of the hole and calls for someone to help pull him out.

Later, Hayd recovers in the medi-cabin and calls for his friend Drew, who's just returned from the mountain with the others. The two friends comfort each other, and then Drew tells Amy that Farl is outside and wants to talk to her. While the others are building a funeral pyre for Eris, Farl tells Amy that he doesn't want to split up with her and he only said the things he did because he thought it might make things less painful for her. Now he realises he was just afraid for the two of them... but Amy tells him there are actually three of them now. He's delighted when she reveals that she's pregnant and they're going to be a band of fugitives from now on.

The Doctor signals Kai to light the funeral pyre, but the old man isn't sure he has the right to do it. Unfortunately the Doctor isn't sure he has the right either as he believes a lot of what happened was actually his fault. They invite Lucie to take the lead and she agrees. Kai looks at the body of Eris and says goodbye. He apologises for what happened, but says that he must have lost perspective after hearing the voice inside his head for all that time. Eris was the best thing that ever happened to him and now he's lost her too. The Doctor agrees that she was an exceptional person with a remarkably strong will. She fought the entity tooth and nail and bought them vital time. Without her, who knows what would have happened? Kai says he can't hear the voice any more, and although the Doctor can't be sure the entity is dead, at least it'll be too weak to try again for a very long time. They agree that Phobos should be an adrenalin-free zone from now on. Kai isn't sure what he wants to do next, but now the entity has gone, Phobos could be a good place once again. Eris loved this place, so he feels he should stay and give something back to it. The Doctor and Lucie say goodbye and leave Kai to his thoughts. Lucie is curious about one thing -- Kai fed the entity the worst fears of everybody in this place, including the things that people think when they're about to die, yet it wasn't enough to stop it. But the Doctor stopped the entity all on his own. He asks her whether she wants him to show her what he showed the entity, but she's not too keen. As the Doctor leads her back to the TARDIS, Lucie wonders whether he might possibly be scarier even than the monsters...