Nigel Smith ran out the door looking for his young wife. 'Alice? Alice, where are you?' His panicked voice met only with silence; she was nowhere to be seen. His mind reeled with possibilities. She was upset and wandered off, or she needed to run errands and forgot to tell him. Or worst case, he'd finally been discovered. He knew his experiments were deemed unethical, but he had to prove everyone wrong; to prove that genetic engineering was not harmful. After all, this was the 1950's, not the stone age.

Nigel saw two people approaching, a tall man with white hair and a slender woman with short brown hair. As they got closer, he walked briskly up to them, shock crossing his face. 'Alice, what did you do to your hair?'

The woman stared at him. 'Sorry? You must have me mistaken for someone else.'

'Don't be ridiculous, Alice, I know my own wife.'

Sarah Jane Smith glanced from the man in front of her to the Doctor. 'I'm sorry, I'm not your wife.'

'Come off it, Alice.' He looked at the Doctor. 'Who are you, sir?'

'I'm the Doctor, and this is Sarah, not Alice.'

Nigel lowered his voice to a hiss. 'A doctor? You went to a doctor? Do you know how dangerous that is? If he knows the truth, this child you're carrying will be destroyed and you know it!'

Sarah defiantly put her hands on her hips. 'Look, I don't know who you are, but my name is Sarah Jane Smith, not Alice, and I am certainly not carrying your child.'

As recognition hit Nigel, he smiled. 'Of course, my mistake! I know your mother, Sarah. It's striking that you look so much alike, I mean, you would, but to see it in front of me! Please, let me make you some tea by way of apology. I would love to discuss how you arrived here.' He leaned in and whispered, 'I'm surprised time travel is possible in your time, you're from around what, 1980 or so?'

The Doctor watched the exchange, knowing who stood in front of them. 'Professor Smith, Sarah and I would be happy to accompany you. I'm sure you have lots of questions for her, and I'm sure she'll have plenty for you as well.'

'Perfect! Follow me, please.' Nigel turned on his heel and headed back towards his house.

As Sarah and the Doctor started to follow, Sarah held back. The Doctor whispered, 'Come on, Sarah. It's just a harmless cup of tea with your father.'

Sarah's concern was apparent. 'But Doctor, my father was a businessman, not a scientist. How could he know about time travel? Something's not right, I don't like it.'

'We need to find out what he's up to.'

'Why does he have to be up to something?'

'Given the nature of his questions, do you think he's not? I'm assuming the baby he mentioned is you, and don't you want to know why he's so concerned about a doctor finding out about you?'

Sarah nodded, 'I suppose you're right.'

In Nigel's house, the Doctor watched Sarah's uncomfortable expression and turned to Nigel, who regarded Sarah with pure fascination. 'So, Professor Smith, I must congratulate you on the success of your research.'

'Likewise, Doctor. I've always agreed with the theoretical principles of time travel, I just didn't think I would be able to see an actual manifestation. To bring Sarah and allow me to see my success in my experiments, it's brilliant, absolutely brilliant! This will finally give me ammunition against Lavinia and all my opposition.'

'We really can't stay long, Professor. According to the laws of time, Sarah shouldn't be here at all.'

Nigel laughed, 'Yes, Doctor, of course. But it seems as you and I are much alike; only heeding the rules unless we know the outcome will be better if we don't.'

Just then Alice walked in the front door, carrying a small bag of groceries. 'I'm sorry, I had to run out, Nigel, but I needed…' she stopped mid-sentence when she saw visitors. 'I'm so sorry, I didn't realise Nigel had company.'

Nigel looked from Sarah to Alice and laughed. 'Look at that Doctor, it's amazing. Sarah is the perfect clone! They are identical.'

Both women looked at Nigel, as did the Doctor. 'Clone?' Sarah exclaimed. 'That's impossible, it can't be! Even in my time that's impossible.'

The Doctor rubbed the back of his neck. 'Ah, yes, well, that's where things have changed, isn't that right, Nigel?'

Alice dropped her bag, grabbing her stomach. 'Help me,' she gasped before passing out.

Nigel was at her side in a second, picking her up and quickly heading down into the basement. 'Doctor, I might need your assistance.'

The Doctor and Sarah followed Nigel into the basement where they saw Nigel's laboratory, painted a sterile white with microscopes sharing space with syringes and Bunsen burners. The one standout in the room was the steel operating table.

Alice gasped for air as Nigel laid her on the table. 'It's like the baby is burning inside.'

The Doctor motioned to Sarah, 'Stand on the far side of the room, it's possible your proximity might be causing some of this.'

Alice calmed down as Sarah moved away. 'It's easing now.'

The Doctor turned to Nigel. 'See what I mean about the laws of time? Some laws are not meant to be broken. Nigel, if you complete this experiment, you will change history forever, and not for good.'

Nigel turned on the Doctor. 'What are you talking about? Sarah is standing right there! She's a perfect copy! The only DNA in her is what I manufactured!' He gestured towards his microscope. 'Here, see for yourself, Doctor. I've genetically engineered Sarah's DNA sequence to fight off illnesses. Why, not even the common cold will affect her!'

The Doctor looked in the microscope and frowned. 'Nigel, look at this again. The sequence is breaking down; it's not stable. If you continue, she'll live two, maybe three weeks after she's born. Her tiny body won't be able to sustain it. You will have brought a life into this world, only to kill it. The burning sensation Alice is experiencing is not due to Sarah's proximity, but the DNA structure of the baby beginning to break down.'

Nigel peered into the microscope. 'That's impossible! It's been absolutely perfect up until now.'

As Alice slowly sat up, Sarah moved over to her. Alice grabbed Sarah's hand. 'Please don't let me lose another baby. I want this child more than anything else.'

Sarah's eyes pleaded more than words could. 'Doctor, please, do something.'

The Doctor started moving around the equipment and found two clean needles. He pricked his own finger, then walked over to Sarah and pricked hers. He talked while working. 'Nigel, allow me to make a couple of changes to the baby's DNA structure. It will save her life. Genetic engineering is a tricky business and human cloning even more so. I can tell that you're a brilliant scientist, but you're about 100 years too early for this. There will be places that can perfect this in time, but right now, you don't have the equipment to make sure any of this is stable. Allow me to save your child, to save Sarah, so that this whole thing doesn't turn into some sort of time paradox.' After a moment, he held up a syringe. 'Please Nigel. Let's leave the genetic engineering for the future, where it belongs. Save your family and protect Alice from any more pain.'

'Doctor, you're destroying my life's work!'

Alice stood up and walked over to the Doctor. 'Do what you need to. This baby is a life and more important than a science experiment.'

'Alice!' Nigel hissed.

'Nigel, you can have your science, but you can't experiment on me or this baby any longer!'

Sarah walked over and took Nigel's hands in her. 'Please, for Alice's and my sake. Let the Doctor help.'

Nigel hung his head. 'All right, Doctor, do what you must.'

The Doctor led Alice back to the operating table. 'This strand of DNA will work its way through the baby's structure and stabilise her, so that by the time she's born, she will be a normal, healthy child.'

Sarah watched as Nigel and the Doctor worked together. After what seemed like only a few minutes, the Doctor turned to her. 'OK, Sarah Jane, let's go. We've stayed here too long already.'

Nigel frowned. 'Must you go so quickly, Doctor? There's so much I want to know about your time, your science, Sarah's future.'

The Doctor smiled, 'Time will reveal all, Nigel. You just take care of Alice and the baby.' He reached out and took Sarah's hand to lead her out.

Sarah released the Doctor's hand and went up and hugged both Alice and Nigel. 'This is awkward, but I'll see you soon.' She quickly followed the Doctor upstairs and out the door, back to the TARDIS.

The Doctor entered the console room as the TARDIS landed. He turned on the scanner and saw the wreckage outside, just as anticipated. The bonnet of the car was still smoking from the collision with the tree. The Doctor held out his hand and examined the syringe before putting it in his pocket and going out the door. He didn't have much time.

Glancing in the front seat of the car, he knew it was too late for the driver and his passenger. He quickly opened the back door and called out to the young girl lying curled up in the back seat. 'Sarah Jane?'

Sarah often dreamt of a double heartbeat. From the time she was a little girl, the double sound echoed in her ears, and she was never able to figure out what it meant or where it came from.

She recognised the pattern the first time the Doctor grabbed her to keep her from falling. The memory jolted her. She vaguely remembered it from her childhood, thoughts struggling against the edges of her consciousness. She woke immediately as the TARDIS landed, fuzzy memories finally clearing after all this time. It was the accident that killed her parents, and the man that pulled her from the car.

As she entered the console room, it was the view on the scanner that stopped her dead in her tracks. The Doctor was outside, walking up to a wrecked car. She gasped when she saw him pull a younger version of herself out of the car. Her dreams suddenly snapped into reality right in front of her. She reached over and turned up the volume to watch more of her own history unfold.

Five-year-old Sarah looked up at the Doctor. Other than a small, bleeding cut over her left eye, she seemed unharmed. 'What happened? Where's Mummy and Daddy?' she asked softly.

The Doctor reached out for her. 'I'm the Doctor, a friend of theirs from before you were born. Come on, we have to get you out of here.' He pulled her out of the car and quickly put a safe distance between them and the car. Sarah held on tightly to him, arms wrapped around his neck. The Doctor knelt down and put Sarah on his knee to properly look at her cut. 'Let's see what we can do about that cut, shall we?'

'OK.'

He reached into his pocket and pulled out an antiseptic cloth and cleaned up her cut, singing soothingly to her 'There you go.' He lightly tapped her on the nose and smiled, his hearts breaking as she gave him a small smile in return. He took the syringe out of his other pocket and administered it to her. 'This will hurt a little, but will protect you from more than you can imagine. I'll give you something to reverse it when we meet again in about twenty years.'

'Twenty years? Where are you going? Are you going to leave me here? Can't you stay with me? Please?' Sarah asked as she put her head down on his chest with her ear over one heart and one hand over his other heart. After a minute of listening, she pulled back and looked up at the Doctor. "I can hear your heartbeat. Do you have two hearts?'

The Doctor laughed. 'Sweet Sarah, you ask so many questions. You will be a terrific journalist one day.'

Her lower lip trembled. 'Where are my Mummy and Daddy?'

The Doctor reached up with a handkerchief and wiped away tears as they spilled over. 'Sarah, look into my eyes. Do you trust me?' After she nodded, he reached up, touched her temples and continued. 'Sarah Jane, you won't remember any of this.'

In the TARDIS, Sarah watched, tears flowing down her face, as the Doctor took his jacket and wrapped it around her younger self, before heading back to the TARDIS. Sarah didn't want to be there when he returned, but she found she couldn't move. 'Oh, I remember now, Doctor. I remember it all.'

The Doctor walked in and saw Sarah, back towards him, facing the scanner, head bowed. 'Ah, Sarah Jane, I, uh, didn't, um, expect you to be awake.'

She lifted her head and gestured back towards the scanner. 'I can't thank you enough. First, my father's experiments, and now, the accident. I owe you my life, again.'

The Doctor closed the gap between them, reached out and tapped her nose, smiling. 'Just consider it keeping your personal timeline in check. Besides, combining genetic engineering with the emerging technology in nuclear physics and humanity would have trouble on its hands.'

Sarah sniffed again and wiped away her tears. 'Thank you. But how did you know?'

'I did some research on your family when something you said about your parents didn't match up. I knew your Aunt Lavinia was protecting you from something, but I didn't know what at the time. Then I recognised the abnormalities in your DNA. The only way to fix it was to go back before you were born and find out what happened.'

'So you knew all along what was going on?'

'Not exactly. It had to play out.'

'I see, I think. So, what did you just give me out there?'

'I re-altered your genetic structure. I couldn't allow my DNA to stay uncovered in your system, so I introduced something that would mask it, prohibiting any doctors from discovering it.'

'So I have Time Lord DNA running through my system?'

'Just the slightest little bit. It shouldn't cause any problems. Hopefully.'

Sarah smacked him in the arm. 'Hopefully? I'm beginning to feel like a lab rat, you know. I don't know whether to be grateful or furious!'

The Doctor smiled. 'Happy Birthday, Sarah Jane.'

'How did you?' she started before the Doctor cut her off with a Gallifreyan version of the birthday song, leaving Sarah holding her ears and laughing.