At three in the morning, Jo has a tendency to worry about things like the state of the planet, Cliff’s blood pressure and - if the Doctor isn’t here any more - who’s saving the world from alien invasions on Fridays? At the moment Cliff and Jo aren’t at home, which is half way up the Rio Negro at the Institute of Mycology, but are instead in London for the climate conference. When they arrived, they both went to look at UNIT’s old HQ, but it’s been turned into an embassy for one of the new east-European states. Typically, Jo is laid up in bed at the hotel while her husband single-handedly harangues the UN‘s World Future Conference about the imminent collapse of the Amazon forest. With nothing better to do, she decides to catch up with her blog.

She recalls the time when she was 18 and was stationed here with UNIT. She remembers the Brigadier getting annoyed with her because alien invasions always happened on Fridays, which meant she kept missing her weekly training course. The Doctor used to refer to it as ’spy school’ and said it was a waste of money, but that just made the Brigadier angrier as he thought it was important Jo learned about surveillance and intelligence work. From then on, Mike Yates always called her ’Jemima Bond - licensed to spill‘ which made everyone laugh, but Jo took the comments to heart and decided to teach them what she was really capable of.

One Friday morning, she was already late for training when the Doctor told her he’d picked up some sort of temporal disturbance. He called it ’blue-shifting’ and told her about retro-causation, which seemed to mean time in reverse. He said it was far more important she help him track down the signals than go on some useless role-playing exercise, so off they went in Bessie instead. After three hours, they’d located the source to the National Museum in Bloomsbury. The Doctor marched inside with the blue-shift detector going crazy, pausing only to look at an ancient Egyptian mummy on display, which he said he used to know.

The signal led them to the office of Professor Harold Sanders, Director of Anomalous Artefacts. He was a tweedy old academic and was taking a lunchtime nap when they burst in. The Doctor accused the man of harbouring a source of dangerous and disruptive energy, but Sanders was furious and Jo had to calm things down. Sanders revealed that he’d been seeing small blue glowing ghosts, like children, but nobody else believed him and everyone thought he was mad. A thought popped into Jo’s head and she asked him if they looked like old-fashioned china dolls. Sanders confirmed that they did, except for one that looked like a teddy bear. The Doctor was intrigued when Sanders said the ghost spoke backwards and he concluded this was no ordinary infestation of ghosts and something must be disrupting the flow of time itself.

The Doctor suspected it was the same thing that had drawn them here too. When he got out his blue-shift detector, there was a sudden burst of energy and all the loose papers on the desk fountained up into the air to reveal a tin hidden underneath. Sanders tried to snatch the tin away, but the Doctor grabbed it and the lid flew off to reveal a hand-sized lozenge of brown glass, flickering with red and blue light. Sanders accused the Doctor of activating it, something he’d been trying unsuccessfully to do for weeks. The Doctor said the object was dangerously unstable and he insisted on running some tests on it back at the laboratory, but Sanders refused to hand the object over to a rival scientist.

Jo persuaded the Doctor to leave and said she could ask the Brigadier to impound the object, but the Doctor wanted to run his tests in private. He asked Jo how she knew what the ghosts looked like, but she didn‘t know. They were accosted in the corridor by a fierce looking woman, who looked a bit like an old hippy with bangles and crystals. She insisted the tablet was her property and said it had been stolen by Sanders. The Doctor asked her where she found it, but she said it found her. She went as white as a sheet when the Doctor told her it was alive and calling for help.

Suddenly there was a crash from the Professor’s office and they ran inside just as a blaze of white light burst out and arced over them. Sanders was slumped over his desk. Climbing on the shelves were three little ghosts, flaring with blue light and when the woman spoke to them, her words came out backwards. Two of the ghosts were dressed in old-fashioned clothes, but the third was a teddy bear, just as the Professor had said. They were the ghosts of toys! Then, just for a moment, Jo noticed a shape lying on the floor and realised it was herself, apparently dead. As the vision disappeared, the Doctor ordered the woman to call off the ghosts. Sanders was dead, but the woman was only interested in finding her tablet. They heard the sound of large angry dogs and the woman said they were Retrievers. The ghostly dolls jumped up into her arms and she ran from the room. A blue light appeared, then three dogs, each the size of a pony and with ugly battered faces and no eyes, ran backwards out of the wall and started searching the room. They sniffed around as the Doctor and Jo froze. The blue-shift detector gave a crackle of energy and the three huge dogs went running backwards out of the door. As they recovered, the Doctor assured Jo the dogs couldn’t have hurt her as their timelines were just passing like ships in the night.

The Doctor and Jo spent a couple of hours helping the police with their enquiries but the Brigadier pulled some strings to get them released. It was concluded that Professor Sanders had died of a heart attack, but the Doctor said he’d been killed up by a discharge of temporal energy. The next morning, Jo found the Doctor in his lab examining a small fragment of the shattered tablet which he found lying under the Professor’s body. He concluded that it was a conduit - a side door to a parallel Universe where time travelled in the opposite direction. He couldn’t analyse it properly without better equipment, but the Brigadier refused to fund any more of his outlandish projects. In fact, he said he was placing a ban on all private work and demanded more co-operation from the Doctor, beginning with a pile of tax returns which the Doctor needed to fill out. The two men started arguing furiously, so Jo took the blue-shift detector and slipped out unnoticed. Spookily, she’d woken up that morning ‘knowing’ that the name of the woman they’d met was Doreen Killebrew, and when she looked in the phone book she’d found an entry for Killebrew’s Toy Hospital in Sago Street, not far from the Museum.

As she left UNIT HQ, she bumped into an off-duty Sergeant Benton and asked him to give her a lift across town in his new car. She told him how she planned to convince everyone she wasn’t just a “dolly bird” by doing some investigating of her own. Benton said the way to keep the Brigadier happy was simply to follow his orders, but that was exactly what Jo didn’t want. They found Sago Street cordoned off, and although there were no police about, the place looked like a bomb had hit it. Before Benton could stop her, Jo ducked under the cordon and headed for the Toy Hospital. The windows of the shop had been blown out and the evidence suggested the explosion had occurred inside. The blue-shift detector became active when Jo entered and she went to the back where the doll surgery was located. Upstairs she found a room filled with Victorian furniture - and there were creepy looking dolls everywhere, all dressed in old fashioned clothes and bonnets. There was also a moth-eaten teddy bear. They didn’t move, but Jo was certain they were all watching her.

Benton joined her and suggested they leave, but they turned to find Mrs Killebrew blocking the stairs! She glared at Jo intensely and demanded to know whether she’d brought the tablet back. Benton asked what happened to the shop, but the woman said the explosion hadn’t actually happened yet and she mocked Benton when he said he didn’t understand. Then Mrs Killebrew suddenly started talking backwards again and Jo got the impression it wasn’t the woman talking at all. Mrs Killebrew told her the Doctor must bring the tablet back, then she pointed to a sad looking doll in the corner and said HannaH needed to get home as she’d been trapped here too long. Then Mrs Killebrew warned them it was too late as they could hear the barking of the huge dogs in the distance.

Mrs Killebrew cowered in a corner as HannaH started crackling with power. Benton realised the doll was about to explode, so he tugged at the door. Every other doll in the room started to glow blue too and they all turned to stare at the intruders. One by one, the dolls started crawling towards Jo and tiny china fingers clawed at her ankles. Benton broke the door open, but the dolls grabbed hold of him and it was all he could do to push Jo to safety as the door closed again.

Jo raced out of the shop and into the street - and found herself looking into the barrels of UNIT’s armed response squad. The street was full of soldiers and she realised Benton must have radioed in when they arrived. The Brigadier yelled for her to get out of the firing line, but she warned them the shop was about to explode. Learning that Benton was still inside, the Brigadier headed for the building - then he stopped in shock as he saw a second Brigadier, standing inside the shop and staring back at him. The image disappeared and the Doctor said it was an example of time inversion. Suddenly the three Retrievers appeared from nowhere, glowing blue and running backwards across the street. As they disappeared inside the shop, one of them dropped a doll’s arm from its mouth, then the little arm flew up off the ground and went through the shattered window of its own accord. The Brigadier fired at the dogs through the broken window.

There was a blazing white flash and fierce jolt, then all the debris that was flying inwards suddenly stopped and exploded back out again. Jo found herself standing next to the Brigadier inside the broken shop, watching duplicates of themselves running backwards through the smoke and back out onto the street. Benton, his jacket torn, staggered downstairs and told them how he’d managed to fight the dolls off. He carried a little severed doll’s arm in his hand.

Suddenly the three massive hounds came bounding out of nowhere, but this time they were moving forwards as Jo, Benton and the Brigadier were now in their time stream. The sightless dogs sniffed around them and then headed towards the blue-shift detector in Jo’s pocket. Benton waved the doll’s arm at the dogs and then threw it out onto the street. The dogs followed the scent outside and they could see the earlier version of the Brigadier raising his gun. Benton grabbed Jo and forced her to the floor as bullets came flying out of the woodwork and then miraculously returned into the Brigadier’s gun. One of the bullets struck Benton and he grabbed at his bloody arm. The Brigadier stepped up to the window, and for a moment he saw his own mirror image on the street staring back at him incredulously.

Jo ran for the door (narrowly avoiding her earlier self as she ran inside backwards) and stepped out into a world of retro-time. She dodged all the reversing vehicles as they moved backwards out of the street, and would have been hit by the speeding Bessie if the Brigadier hadn’t pulled her away in time. They returned to the shop and Jo treated Benton’s wound while the Brigadier tried in vain to contact UNIT HQ. Benton pointed out that Mrs Killebrew was still upstairs and the Brigadier said she’d been on the phone to him earlier, complaining that the Doctor had stolen something of hers.

They went upstairs to confront her and found Mrs Killebrew on the floor, clutching HannaH to her chest. She started talking in a high-pitched doll-like voice and said HannaH spelt her name with capital letters at either end to represent the fact that she could see both ways. The Brigadier asked what was going on and the woman told him they were all trapped together. Two of the dolls came to life and shut the door. Jo asked who HannaH was and, speaking through Mrs Killebrew, the doll said she was an Observer, a social historian from an alternative Earth running side by side with this one but in the opposite direction. HannaH said her body was still in the other world, but she projected her mind across so she could study this world - but then her lifeline was torn apart when the tablet, her means of contacting home, was stolen. She was left spiralling backwards through an alien time, so she clung to the first stable thing she could reach - the doll. She said Mrs Killebrew enabled her to move and see, but the Retrievers were sent to hunt her down and drag her back. This confused Jo as HannaH had said she wanted to go home, but the doll didn’t respond.

The Brigadier decided they needed to return to their own timeline before they disappeared up their own birthdays. He suggested they get back to UNIT and consult the Doctor. They returned to the street, only to discover Benton’s car wouldn’t work in reverse time, so they had no choice but to walk all the way. The four of them, plus HannaH, headed off down the side streets, but it was like being on the dodgems as they had to avoid bumping into people going about their business backwards.

They could hear the Retrievers not far away and the Brigadier realised they were sitting ducks out in the open. In the hope of shaking the dogs off, they headed instead for Oxford Street where the shoppers were out in force. They soon mastered the art of walking with groups of people who were moving backwards in their direction, but every time the shoppers stopped to look in a window, there’d be a collision. Even Jo found herself being distracted by some groovy boots in a nearby boutique. Ducking in and out of shops, they got as far as Oxford Circus before they saw one of the Retrievers on the other side of the road. As it moved towards them, the Brigadier ordered everyone to run. Fortunately a number 8 bus cut between them and the dog and they managed to clamber aboard. Unfortunately, the bus wasn’t moving very fast and the dog was able to keep up with them easily.

As the bus slowed down for the next stop, the dog snapped at Benton’s trousers and dragged itself on board. The others held onto him as tightly as they could, then Mrs Killebrew delivered a hefty blow to the dog’s head with a rolled up newspaper. The Retriever fell back onto the road with a yelp and swiftly vanished under the wheels of a speeding taxi. The group got off at the next stop, but by now Benton was losing a lot of blood and Mrs Killebrew was exhausted. They went down a side street into a deserted mews - only to find themselves trapped between the two remaining Retrievers. The Brigadier open fire, but the dogs hardly blanched. Protecting the women, the Brigadier and Benton prepared to make a fight of it…but then the Doctor miraculously stepped out of a crack of light and asked Jo for his blue-shift detector. He waved it at the dogs, but one of them lunged towards him and he dropped the device. Jo picked it up and threw it straight into the crack of light. The Doctor protested, but it was too late and as both Retrievers followed the device into the crack, it closed behind them.

The Brigadier and Benton congratulated Jo, but the Doctor was furious. He explained that he got here by reversing the principles of HannaH’s linking tablet so that it tracked down the signal of his own detector, like opening a side door on the passage of time. Mrs Killebrew, speaking for HannaH, demanded that he hand over the tablet, but he said he wanted to keep it for a bit longer. Unfortunately, now that they didn’t have the detector any more, the bridge back to UNIT HQ was closed to them, so they’d have to continue on foot. He suggested the Brigadier take Benton back to UNIT HQ for medical treatment and asked him to meet them at the National Museum yesterday evening, in time for when this all started.

After the Brigadier and Benton had gone, the Doctor and Jo sat down with a weary Mrs Killebrew. Through the old woman, HannaH explained to the Doctor that she came here to study Earth‘s first contact with alien life. To his surprise, she said she didn’t mean the hordes of aliens queuing up to invade - she meant him! He was the one continuing factor that helped see off all impending disasters. For a moment he was flattered, but then he warned her she was wearing out Mrs Killebrew’s body and he wondered how much longer she could endure it.

The Doctor handed over the shards of broken glass and said it was all that remained of the tablet. It still retained its properties, but it wasn’t as strong as it once was. However, now they were going back in time, tomorrow will be yesterday so they’ll be able to go to the Museum and wait for the tablet to implode back into its original form. It was obvious Mrs Killebrew was too weak to go on, so Jo offered to act as HannaH’s eyes and ears instead. She clutched the doll tightly, then she went numb all over and started speaking with the voice of the alien.

When it got to eight in the morning (heading in the direction of half past seven), the Doctor, Jo and Mrs Killebrew found it easier to travel as there were less people on the streets. The Doctor ducked into a café on Baker Street and took some bacon sandwiches before the cook could unmake them. Then they found a place on the bandstand near the lake on Regent’s Park where they could spend the night, hopefully undisturbed. Jo nodded off and when she woke again, it was dark. She was shocked to discover HannaH had been conversing with the Doctor through her body, even while she was asleep. Jo could feel HannaH was angry because the Doctor had refused to give her an interview and she dismissed him as just another patronising male who’d allowed himself to become part of the Establishment. The Doctor argued that the people on Earth were generally very selfless, but HannaH clearly regarded them as narrow-minded and tedious. The Doctor asked her if she knew how much longer his exile would last and HannaH suspected he was after her tablet for himself. They began arguing again and then ended up sulking in silence for hours until yesterday finally arrived and the setting sun began to rise.

The Doctor asked how Jo was, and Jo was a bit disturbed that HannaH spoke on her behalf, rather than let her speak for herself. They realised Mrs Killebrew was no longer with them and the Doctor guessed she must have gone home. They set off at seven in the evening and arrived at the Museum around six. The police were still there and they watched as the earlier versions of the Doctor and Jo walked backwards from Bessie, then went inside for their questioning. After a long wait, the police backed out of the building and then screeched away in reverse, their sirens wailing. The Doctor led them into the Museum - and straight into the Retrievers’ trap. The two remaining dogs came bounding out from amongst the exhibits and charged towards them. The Doctor defended Jo and HannaH with a medieval pike, but they were driven into a corner. Suddenly the doors crashed open and Mrs Killebrew entered, hauling the giant teddy bear from the Toy Hospital. HannaH laughed and then used part of her consciousness to animate the bear. The dogs attacked the bear, and although their bodies were repeatedly hurled against the wall, the bear was savaged wildly, bleeding sawdust. By the time the teddy bear had been ripped to pieces, the Doctor had already used the opportunity to slip into Professor Sanders’ office.

Sanders was still there, slumped dead on his desk. The earlier versions of the Doctor and Jo were there too, as were the Retrievers which then reversed back into the wall the way they’d first come. Everything that had happened before was happening again - only in reverse. Using Jo’s body, HannaH reached out for the tablet under Sanders’ body. Then, to Jo’s horror, HannaH revealed that she intended to send Jo’s consciousness back to the other Universe in her place, so she could remain here, safe in her body. Jo began to fight for control, and although she felt as if she was being torn in two, she refused to give in. Eventually she managed to push HannaH out, so the alien went instead into Mrs Killebrew’s body. Jo fell to the floor, unable to move.

Jo knew the tablet was about to implode, but she was too weak to move. Suddenly the Brigadier burst in and shielded her with his own body. A blaze of white light flooded the room as the retro-explosion dwindled in. They were still going backwards and could now see Professor Sanders, resurrected at his desk. Driven by HannaH, Mrs Killebrew reached out for the tablet - only to discover it was the broken version. The Doctor now had the restored version in his pocket, having switched it with the other one earlier. HannaH demanded he hand it over, but he said she’d done enough damage already to Jo and Mrs Killebrew.

The two Retrievers emerged again from the wall, but then they paused as if surveying the situation. HannaH again accused the Doctor of wanting the tablet for himself so he could escape his exile. The Brigadier was suspicious and reminded the Doctor of his duty here, but the Doctor pointed out that filling in tax returns was hardly rewarding. Jo pleaded with the Doctor not to leave them…and then the Doctor revealed that he had no intention of using the tablet to leave. Mrs Killebrew suddenly clutched her stomach and then sat down with a look of abandonment on her face. The HannaH doll became alive again as the consciousness of the alien returned to it. The Observer thanked Mrs Killebrew, then it asked the Doctor to help it go back to its proper Universe. The Doctor handed the tablet to Jo and said the choice should be hers. In her hand, the tablet started to flicker and glow. She knew she had to make a decision quickly as the past was catching up with them. She decided everyone should make the future for themselves, so she threw the tablet in the air and both the HannaH doll and the Retrievers leapt towards it. As they all collided, time arced around them, fusing and cancelling itself out in a blistering dazzle of light.