Hello! This story is a follow up to 'A Letter from the Past', but it works fine as a stand alone. Enjoy!
Disclaimer: As ever, I own nothing. This fact saddens me greatly. SJA and Doctor Who are the property of the BBC

The Doctor wasn't at all sure why the TARDIS had been pulled down to this particular place and, frankly, he was worried. The time machine was only ever pulled down to somewhere when something serious was happening. He checked his location and time on the monitor: Ealing, 2011. Taking a deep breath, he stepped outside.

At the familiar sound of the TARDIS engines, Sarah Jane and Luke ran outside. Rani and Clyde had evidently heard it too as they had come outside as well. All of them stared at the blue box, waiting for something to happen. Eventually the door opened and a man stepped out whom the teens didn't recognise. Sarah Jane, however, recognised him instantly. She ran forward and threw her arms around him. He spun her round, both laughing with delight.

"Doctor," she cried. "I didn't think I'd see you again. Not like this."

"Oh, Sarah," he sighed. "I have missed you."

"Why are you here?" she asked. "I'm assuming this isn't a social call."

"The TARDIS was pulled down," he explained. "I had hoped you would tell me it was you and that nothing is going on."

"Sorry," she shrugged. "It wasn't me. But I have a way we can find out what it was."

She extricated herself from the Doctor's embrace, her mind buzzing with possibilities and excitement. The three teens were staring at them in confusion. True to form, Clyde was the first to speak.

"What's going on?" he demanded. "And who's he?"

"This is the Doctor. My Doctor," she explained calmly. "Doctor, this is my son Luke and his friends Clyde and Rani. Now come on. I've something to show you."

She grabbed the Doctor's hand and led him to the house and upstairs to the attic. He gazed around, clearly impressed.

Smirking slightly, she called "Mr Smith, I need you."

"How may I be of service, Sarah Jane?" the computer asked. "I detect that your heart rate is abnormally high. Are you unwell?"

"I'm perfectly well, thank you," she said. "Scan the area for any unusual activity please."

She looked at the Doctor. "Impressive, Sarah," he remarked. "I would expect nothing less."

"Sarah Jane, I have detected an abnormal energy trace at the Palmar observatory," the computer announced.

"What sort of energy trace?" she asked, suspicion prickling at her insides.

"It is reminiscent of the Mandragora Helix, although this trace is much weaker."

"What's the Mandragora Helix?" Luke asked.

"It's an ancient energy that seeks to command the universe," the Doctor explained. "It uses other beings as tools and then discards them. It only cares for power."

"But we've beaten it before," Sarah Jane cut in. "And we can do it again."

The observatory had not been hard to find. They arrived to find all the lights on, but there was little sign of activity. Cautiously, they entered the building.

"Not another step," a voice from the shadows warned. They found themselves grouped together with a gun pointing at them. The man wielding the gun was rather short, had thinning grey hair and was sporting an alarmingly loud jumper.

Out of nowhere, a voice spoke. "I know the Doctor is here. Bring him to me."

The group were roughly shepherded down a narrow corridor.

"Doctor," Luke hissed. "What's going on?"

It would appear that the Helix requests my presence," the Doctor replied. "I should imagine that it needs me for something." He managed to project an air of calm, but strong grip on Sarah's hand told another story. She gently squeezed his hand in what she hoped was a reassuring way.

By this time they had reached a small office in which there was a desk and a computer. There was a bright purple wave pulsing across the computer screen which increased sharply in frequency upon their entrance.

"Hello, Doctor," the computer said in a menacing tone. "So glad you could make it. And with all your friends too."

"What do you want, Mandragora?" he demanded.

"Revenge," the Helix replied abruptly. "I am merely a remnant of the energy used in the last attack on this worthless planet, but I have grown stronger. Strong enough for this."

The whole group watched in frozen horror as the energy left the computer and entered Sarah Jane.