Ace is delighted to be home at last, and even the Professor agrees that they’ve come via a slightly circuitous route. It feels like she’s been away for ages and she enjoys showing him the streets and parks around the tower block where she lives, even though it all seems so much smaller than she remembers. Excitedly, she takes him to the community centre, but when she calls out there’s no one about. It’s not exactly the welcome she had in mind, as she was looking forward to impressing her friends with amazing stories and giving knowing smiles and obscure answers that say nothing, but hint at everything. She decides to try other places, but there’s no one at the flats, the nearby park, or even the pub. The Professor reminds her that time and people move on -- but just then, she sees her old friend Mischa across the street. She races out into the road, calling her name...

Suddenly, there’s a screech of tyres and the Professor shouts in alarm as Ace is struck by a passing car and thrown to the ground. The driver gets out and immediately protests that she ran out in front of him and that he couldn’t stop in time. As the man calls for an ambulance, the Professor urges his young friend to hold on, but Ace is too busy rebuking the driver for nearly hitting her. The man continues making his call and Ace becomes angry at being ignored. She goes over to the Professor and is shocked when her hand passes straight through him. Before she can react, an elderly woman appears in the shadows nearby and tells Ace that no one can hear her. Ace asks the woman to step out into the light and is amazed to see that it’s her Nan! She can’t believe her eyes as her Nan is dead, but then the woman gently points out that Ace is dead too...

Ace is convinced that she’s dreaming and that all she has to do is close her eyes and count to three... but it has no effect. Nan shows her the Professor, who is currently kneeling on the ground by Ace’s body. She says Ace’s “essence” has already left her body, which Ace thinks is like an out-of-body experience, but Nan says she’s wrong. An out-of-body experience is what happens when part of the essence still remains to anchor the person to this world, but in Ace’s case, that essence has been detached completely, so it can’t go back. Nan has come here to help Ace with the transition. Refusing to believe it, Ace calls out to the Professor again, but by now the ambulance has arrived and the paramedics are doing everything they can to bring Ace back to life. Frustrated and angry, Ace accuses her Nan of being behind everything and can’t accept that the woman is who she claims to be. Nan explains that she died ten years ago after falling down the stairs and breaking her neck. It was Ace who found her body two days later. Nan remembers how brave she was, calling the police and the ambulance, even though she was shaking like a leaf. Ace begins to suspect that she’s hallucinating and that Nan is a figment of her imagination, which makes Nan smile as she always brought Ace up to be a doubter, to look at the world with a cold eye. When Ace asks her if she’s really an alien, the woman just laughs. Ace is distraught when the Professor gets in the back of the ambulance and disappears with her body. She runs after him...

...and then suddenly finds herself in limbo. Nan tells her this is the point between her old existence and being on the other side. Ace thinks the ‘other side’ must mean Heaven, but Nan says it has lots of names. Every culture has a different word, but they all mean the same thing really. Ace tells her she’s talking crap, which angers the woman at first, but then she sympathises and says she understands her granddaughter’s confusion. She rejects what’s happening because all her senses tell her she’s still alive. Nan did exactly the same thing when she died. When there’s a sudden death, a person’s “essence” still feels a need to resolve things that were left unsaid or undone. Nan kept going back to Ace’s mother when she died and she knows they both had a terrible time coming to terms with their loss and until Nan accepted she had no place in the world any more, her consciousness wouldn’t let go either. Consciousness, soul, spirit and ghost are all just terms used by people who are terrified that death is the end, but Nan assures Ace it isn’t anything like that.

Nan finds the Professor interesting but thinks Ace only latched onto him because she needed a father figure. Ace assures her that isn’t true, then asks Nan to prove her story by telling her what happened to her and her mother after her Nan died. Nan remembers watching as they both became very depressed. Ace cried herself to sleep every night for months, then binged on junk food until she was so sick her friends Mischa and Stevie thought she had anorexia. She bottled everything up and dealt with it by getting into trouble. It was only Mischa who dragged her back to life and kept an eye on her. Even when Nan was still alive, she saw Ace’s mother had nowhere to turn when her husband left except the bottle, and she spent days in bed sleeping her life away. Ace wonders why Nan didn’t say any of this at the time, and Nan admits that she was bitter, but always thought she’d had time to make up for it. Ace has grown up a bit since then and now she can see things more clearly. She knows Nan never really liked her dad. In any case, the fact that Nan can probably read her mind doesn’t prove her story is true. Nan reminds Ace that when she was young, she used to wake up in the night thinking her ghost was in the room with her. That was real and she was there all the time, trying to get Ace’s life back in order again. Ace becomes defensive when Nan questions the Professor’s role in her life, but Nan is beginning to think he might be the best way to convince her to move on. She decides to do exactly what the Professor does himself -- now they’re free of their corporeal states, they can go anywhere and see anything. Nan warns her that she might feel disorientated for a while, but they’re about to see something unpleasant...

...Ace and Nan disappear and suddenly find themselves in the local hospital. Nan can remember Ace coming to visit her once when she came in for an operation. Before they have a chance to move aside, a porter runs straight through them, pushing a trolley. Ace is horrified -- she could see the man’s brains and everything! Nan reminds her she’s non-corporeal and she doesn’t exist in this world any more, so no one can see, hear or feel her. She takes Ace to the reception area, where they see the Professor sitting alone at the back, crying. Ace is desperate to comfort him, but there’s nothing she can do and she accuses Nan of making her suffer just to teach her a lesson. Nan says the only way she can help her to understand is to let her experience it for herself. They watch as a representative from the hospital invites the Professor to follow him.

Ace and Nan visit the Chapel of Rest, moments before the Professor gets there, and Ace is shocked to see her own dead body on display. This is where Ace came as a child with her mother to see her Nan after she died. Quietly, the Professor enters and crosses to Ace’s body, telling her how sorry he is. Ace wants him to know that she’s all right, but he can’t hear her. Nan points out that the Professor won’t touch her, which means he’s already starting to let go. But he’s had to do this before. Ace is surprised when Nan tells her she wasn’t the first person to leave the Professor, and she won‘t be the last. He learned about death a long time ago and he knows how to distance himself. Nan says he only cried because he broke his own rules and allowed himself to become too attached. The Professor whispers to Ace’s body, telling her she’s travelling on her own now to a place where even he has never been. He wonders what it’s like. Ace calls for him not to leave, then begins to break down, desperate to convince herself this is all an illusion and that she’s still alive somehow. She begs him to come back, but Nan says he can only go forward.

Ace and Nan disappear and reappear again in limbo. Nan says she’s only an observer and it’s actually Ace that keeps bringing them back here. She urges Ace not to make things any more painful than they need to be, but pain is the only thing telling Ace she’s still alive. Nan thinks the pain is also an illusion and it’s only her own will that insists that she still exists. Ace refuses to listen and repeats over and over that she’s alive.

Suddenly Ace and Nan vanish and reappear outside a church. In the graveyard, Ace’s own funeral is taking place. When they return to limbo, Ace wonders why she didn’t see the Professor there, but Nan tells her he’s already moved on. Ace doesn’t believe this and remembers when he spent weeks looking for her after she went missing recently -- but this time the Professor knows she’s dead, so there’s nothing to keep him here any more. Ace begins to wonder if Nan has been telling the truth all along. She asks what’s going to happen next and Nan says that depends on her. She will eventually cross over to the other side, so it just depends how long it takes Ace to give up her past existence. Nan is aware that she’s travelled in time and space and gone to other worlds and met aliens, so she encourages Ace to think of crossing over as just another step in the same journey. Ace asks what it’s like on the other side and Nan says it’s a higher, better place, filled with light and joy and wonder. She’ll get to meet all the people she’s loved and lost and it’s unlike anything she could ever imagine. Ace finds her answer to be too vague, and it reminds her of the Professor when he’s up to no good. She challenges her to prove it’s true, but Nan counters this by saying Ace can’t prove it isn’t true either. Just for a second, Ace hears the Professor’s voice, urging her to hold on and stay with him, and she can see his concerned face looking down at her. He disappears and Nan claims not to have seen or heard anything. She says it must have been an illusion, Ace’s last desperate attempt to cling on to her old life. She warns Ace that if she doesn’t let go, she’ll find herself re-living the moment of her death over and over again, because it’s the only moment she has left now.

Ace becomes tougher and refuses to give up. She wonders if she can find the Professor, just as Nan did earlier, but Nan says all she’s doing is prolonging the pain and eventually it will become too much for her. She’s glad Ace found such a good friend, but believes it was at the expense of her own family. Ace says he was the one who believed in her and stuck with her when nobody else would. She laughs when she remembers the first time they met, when she thought he was just a weird little goon and she didn’t even know whether she liked or trusted him, but he was the first person to treat her as an individual rather than just a teenager. He never judged her or told her what to do, and when she made a mistake he never made her feel like a stupid kid. He had faith and saw something in her that even she couldn’t see. She realises she’s starting to sound like a woman from an old black and white film. Nan tells her she’s sorry -- for what Ace is about to do to herself.

Ace and Nan are suddenly whisked away and although they don‘t recognise where they are, Nan tells Ace this is the place she most wanted to be. They hear the Professor’s voice urging someone to keep up with him -- but his companion isn’t Ace. A young girl asks him where they’re going for their holiday and the Professor tells her Great Yarmouth can be quite charming at this time of year. The girl was thinking of something a bit more exotic with sunshine and good-looking men, like one of the Leisure Planets he’s always telling her about. As they pass by, Nan says the Professor was never going to stay alone for very long. He’s the kind of person who needs companionship more than even he knows. The girl is called Nadia and when her parents were killed, the Professor rescued her from some terrible orphanage. Ace dismisses the girl as a three-bottle blonde, but Nan tells her she has no right to be jealous. Ace is shocked as they’ve only just buried her, but Nan tells her that was over six months ago. Time has flown by for the Professor and he’s got on with his life. Ace can’t be part of that any more.

They disappear again and find themselves in a desert -- the Professor and Nadia’s next destination. They watch as Nadia warns the Professor, who’s sitting in his deckchair with only his umbrella for protection, about the dangers of heat stroke. It seems he promised her sun, sea and sand, but there’s no evidence of any sea unless the tide has gone out all the way to the next continent. He admits they may have taken a wrong turning so she starts to nag him to take her somewhere else. When he falls asleep, she storms off in a huff. Nan wants to leave, but Ace feels Nadia isn’t looking out for the Professor properly. Nan tells her there’s nothing she can do about it and accuses Ace of being stubborn. Ace reminds Nan that she always said she should be allowed to make her own mistakes and she doesn’t see why this should be any different. Nan just wants her to be free from the pain of this non-life where she‘s unable to breathe or touch a flower or feel the sun on her face. All these things are gone now, but she can still cross over to a better place. Ace refuses to leave until she’s explained properly. Nan says she can’t unless there’s some common ground between them, but Ace dismisses this as mumbo-jumbo.

They disappear again and find themselves by the Citadel of Lights on the planet Mino. Or at least that’s where the Professor was supposed to take Nadia. In fact, they’re in another rotten gloomy old tunnel and Nadia rejects the Professor’s assurance that darkness is all in the mind. Ace and Nan watch as the two travellers hear a noise ahead that sounds suspiciously like a big hairy something looking for its dinner. The Professor suggests they adopt Plan B, but before he can say “run”, Nadia has already sprinted for the exit.

Ace and Nan return to limbo and Ace demands to know whether the Professor is all right. Nan notes that Ace isn’t concerned about Nadia’s safety, but Ace says she doesn’t trust Nadia to watch the Professor’s back. She wants to go back to the cave, but Nan tells her there are limits to what they can do. She must choose between crossing over or living through the purgatory of staying here, impotent and trapped in limbo, neither dead nor alive. Ace realises Nan has made a mistake -- she’d previously told her she was already dead. Nan tries to backtrack, claiming she was talking figuratively, and insists that it’s time Ace came with her. Ace refuses and tells Nan to leave her alone, so the woman disappears. Ace is now in complete darkness. She calls out, but there’s no response. Eventually Nan returns, warning Ace that this is what she can expect if she wants to be alone.

They return to the cave where the Professor and Nadia encountered the animal. Nan explains that it was a mythical beast, but admits that she doesn’t know as much about such things as Ace does herself. They watch as the Professor and Nadia prepare to leave the planet and walk through a procession of applauding people. Nadia is proudly showing off the cloak she was given by the inhabitants and the Professor jokes that it’ll be a big hit in Carnaby Street. The two of them leave, laughing.

Ace and Nan return to limbo. For the first time, Ace is starting to realise that the Professor doesn’t need her any more. She asks Nan what she needs to do to cross over, and Nan tells her she just has to make the decision. She’s the only one keeping herself here, but that will change as soon as she lets go. Ace begins to cry and Nan knows that it’s hard for her, but once she agrees to cross over the pain will go. Ace contemplates sticking around just a bit longer, just long enough to get her head round things, but Nan tells her that’s exactly what she told herself all those years ago. She’d hoped Ace wouldn’t put herself through the same agony she went through. The loneliness is destructive and it eats away at you from the inside. Ace breaks down completely and Nan encourages her to let it all out.

By the roadside, the paramedic tells the Professor to keep Ace’s head steady. The blood pressure is good and there are no broken bones, but her pupils are fixed and dilated. Suddenly there’s a glow all around Ace’s face and the paramedic is confused...

Ace tells Nan she’s seen the terrible effect of loneliness. Not long ago, she met an android called Dorsai. When his creators died, he was alone for a hundred years. He was sweet, but the way he focused on Ace was frightening. He was a machine, yet he was alive, and if it wasn’t for the fact that he didn’t have any emotions, he’d have gone mad. He had no desires, but he still had a need for the company of others. It was only half a life, and in the end it destroyed him. Nan gently tells her it’s time to go, and Ace agrees.

The Professor tells the paramedic that Ace’s mind has been infiltrated by some kind of malevolent alien entity. He has to find some way of reaching her and encouraging her to fight back. The paramedic tells the Professor not to touch the girl, but he says he knows what he’s doing. He concentrates and whispers to Ace, urging her to focus on his mind...

Nan tells Ace that all she has to do is step into the light... but Ace can still hear the Professor’s voice calling for her. Nan insists it’s an illusion, but the Professor continues urging Ace to fight the intruder, assuring her that she’s still alive. Nan grabs hold of Ace and orders her to stop fighting, but as they struggle, the truth finally hits Ace. This isn’t her real Nan. The old woman drops her pretence but tells her she’s here to help. They come to everyone at their moment of death and the light is a vortex, a transit to the other side where Ace can live on a higher form of consciousness beyond corporal existence. Ace demands to know why she was pretending to be her Nan and the woman says that presenting the image of a loved one makes the transition more comfortable and easier to accept. The Professor is still urging Ace to fight, but Nan says it’s just a delusion, borne of Ace’s desperation. Ace refuses to accept that and Nan says she’s never known anyone to be so resistant to the inevitable. Ace dismisses the woman as a parasite, preying on people when they’re at their most vulnerable, and she now realises the transfer won’t work unless she gives in willingly. She realises this limbo place is a prison and the woman is trapped here, between the dimensions of the corporeal and the non-corporeal. The woman may be neither alive nor dead, but Ace’s body is still out there -- perhaps it’s her body that the woman wants?

Nan decides that she’s been patient enough and starts to physically change into a demon-like creature, but Ace has seen it all before and is far from impressed. She guesses the monster has been here for hundreds, maybe even thousands of years, always tempting people with promises of life on the other side and then trying to take over their bodies. But she must have always failed. The monster lashes out and punches Ace, but it can’t destroy her as it needs her consciousness to cross over. However, it tells her that if she doesn’t agree, her mind will die, and when that happens, her body will die too. When it adds that there will always be others it can convince, Ace decides to go on the counter-attack and physically strikes the monster, then scratches it with her Blue Peter badge. She realises the creature can feel pain too, so if she can destroy it, she can escape -- but the creature says if it dies, Ace will be trapped here forever, whereas at least it will be freed of the torment of unending emptiness.

By the roadside, the paramedic insists Ace goes to the hospital, but the Professor is certain there’s nothing they can do to help her there. He tells him this is no ordinary accident and the paramedic realises he was serious when he said there was an alien presence inside her...

Ace tells the monster it can’t win, no matter how many times it tries. Nobody will ever agree to help it as there’s a part of human nature that will always resist. The monster refuses to accept that and moves in for the attack...

...and by the roadside, Ace’s body goes into convulsions. The Professor pleads with Ace to keep fighting, and then moments later, Ace’s eyes open. The paramedic is shocked to see that she has alien eyes, and the Professor realises things are worse than he thought...

Ace returns to limbo, feeling as though she’s been pulled inside out. She still insists that she’ll never agree to the monster’s request, but it still has another trick up its sleeve. A second version of Ace appears from out of the darkness and stands beside the monster, offering to give the real Ace a good kicking. The doppelganger knocks the real Ace to the ground, then dismisses her as weak and stupid. The monster tells Ace this is part of her own human nature finally unchained -- her hate, filled with a desire to destroy, the screaming craven child fed by her own prejudice and loathing. The doppelganger announces that the first thing she’s going to do when she gets out is dump the Professor. What a self-righteous pain he is! Who is he to decide what’s best for other people? Why should she waste her time looking after some sad old git? The real Ace insists that the Professor cares about her, but the doppelganger believes she’ll be better off on her own. She decides she’s going to team up with Stevie as she’s always fancied him and he‘s got a nice bum, amongst other things. If she can’t find him, she’ll crash a party, get herself some drinks and a couple of blokes who want some fun.

The real Ace assures the duplicate that she won’t make any friends that way. All she’ll get is a reputation, and then people will start using her. The doppelganger doesn’t care and thinks that’s just a fact of life. She’s angry that the real Ace thinks she owes the Professor anything -- after all, if it hadn’t been for her, someone would have put a bullet in him ages ago. She dismisses the Professor as a little boy, stirring up the red and black ants just to keep himself amused. He only keeps people around him as pawns in his game, to stop himself getting bored. Knowing that the doppelganger knows the same things she knows, the real Ace assures her other self that the monster is never going to keep any of the promises it made. Nevertheless, the doppelganger still plans to bond with it and carry the monster inside her. With their shared knowledge, she plans to get everything she wants -- a house, cars, money and men. The two Aces start to argue about which of them the Professor would rather have travelling with him, but the truth is that he needs the complete Ace, the good side and the bad side. The doppelganger claims she doesn’t need anyone, but the real Ace reminds her of Dorsai’s words about having half a life. To simply exist is not the same things as being alive. If the doppelganger bonds with the monster, she’ll just become its arms and legs and it will do what it wants to do, not what she wants to do.

The monster has heard enough and punishes Ace with a burst of energy. She orders the duplicate Ace to take her hand as the bonding can only occur in a corporeal state. The real Ace asks the monster what it’s going to do once it’s become corporeal again and wonders what it did to become imprisoned in the first place. She’s answered with another burst of energy from the creature. By now, even the doppelganger has started to distrust the monster and the real Ace realises it’s not in the duplicate’s nature to trust anyone. The creature starts to moan. It’s waited for centuries, for longer than Ace can comprehend, existing in this nothingness -- still sentient, but living in a void without any sensation. The creature yearns to breathe and to feel again. It asks the two Aces to help it and they realise that it’s genuinely in pain. This is something they can truly understand -- empty lives with barren promises and no hope. Ace is almost tempted to help, but the duplicate version urges her not to listen. The creature begins to plead, but then the doppelganger races across and attacks it...

...and on the roadside, there’s a massive burst of energy. The paramedic leaps back in shock, but the Professor is delighted when Ace revives and calls out his name. She tells him about the thing in her head that pretended to be her Nan, and the Professor says it was a mind parasite. He saw the alien inside her make a rather dramatic exit, which confirms Ace’s theory that she’d got rid of it, but the Professor isn’t sure where it’s gone. Ace then wonders whether it was just the energy dissipating when she broke free -- in which case the monster might still be trapped in the void. They’ll probably never know, but the important thing is that Ace is safe and back amongst the living. Ace wants to leave, and when the paramedic insists she go to hospital, she refuses point blank.