Ace walked into the console room and watched as the Doctor concentrated intensely on the controls. After what she felt was a satisfactory amount of time without his acknowledgement, she cleared her throat loudly to get his attention.
The Doctor simply glanced up at her, 'Oh hello, Ace.'
Ace knew immediately from his tone that something was off. She walked over to stand next to him. 'What's wrong, Professor?'
'The vortex stabiliser is on the blink, so the TARDIS is drifting across timelines that she should never cross.' He furiously punched a series of buttons before the entire room began to shake violently, causing them both to grab tightly to the console to steady themselves.
A small explosion burst from one of the console panels, causing smoke to billow into the room.
The time rotor slowed to a halt, accompanied by it's standard wheezing, groaning growl. The Doctor looked over to Ace before walking around the console towards the still smoking panel. 'You all right?' he asked.
Ace nodded. 'Rough landing.'
'Indeed.' He examined the console. 'Blew out the Gulag oscillator. Hopefully we've landed somewhere I can either pick up a new one or at least the parts to repair it.' He reached up and tweaked Ace's nose before turning on the TARDIS scanner.
Ace turned around and leaned back against the console, crossing her arms as she looked at the image on the screen. 'It's a house.'
'So it is.'
'Why did we land in a house? You won't get your parts here.'
The Doctor made his way towards the door, but Ace didn't move, her eyes locked onto the image in front of her. 'I'm sure the TARDIS has her reasons. Are you coming?'
She gestured up towards the scanner. 'There's one thing this house isn't, Professor. It isn't empty.'
He turned around in time to see what Ace was seeing. The room on the screen had an inner door, and as they watched, the inner door opened and someone stepped into the room. 'Oh,' he said, his voice low as he recognised the woman on the screen in front of them.
Ace turned in time to see a smile spread across the Doctor's features. 'You know who that is?'
'I do.' He continued to smile as the woman on the screen seemed to be waiting for them to exit. Her hands were on her hips and she looked towards the doors. 'My Sarah Jane,' he whispered, almost reverently as he grabbed his hat from the top of the time rotor and dropped it on his head before turning and walking out the door.
Sarah heard the familiar noise of the TARDIS materialisation from downstairs. She steeled herself as she headed towards the attic, knowing that an appearance by the Doctor always seemed to correspond with trouble. The last time she'd seen the Doctor had been their encounter with the Shansheeth and his supposed funeral. It wasn't a set of emotions she had any desire to go through again.
She smiled as the TARDIS doors open, but couldn't hide her surprise when an unfamiliar version of the Doctor emerged, a teenage girl wearing a bomber jacket covered in badges behind him. Sarah absently noted that the girl didn't appear much older than Luke. Taking in the Doctor's expression, she instantly knew something was wrong.
The Doctor raised his hat and gave her a tentative smile. 'Hello, Sarah Jane.'
'Hi.' Her tone was reserved as she waited, hoping he would give her more information so she could determine exactly what was going on.
The young woman behind the Doctor cleared her throat. The Doctor absently turned to her. 'Ah yes, Ace, this is Sarah Jane. Sarah, this is Ace.' Ace and Sarah exchanged hellos, but then an awkward silence filled the room.
The Doctor looked around. 'This is really nice, Sarah. Where are we?'
Sarah tilted her head slightly, regarding him. 'My attic in Ealing.' She crossed her arms, realising that he should know exactly where they were. Unless he'd crossed his timeline. 'How long has it been since you've seen me?'
The Doctor instantly tensed at Sarah's question. 'When I got called back to Gallifrey. You know something, don't you, Sarah?'
'I think I do. You've crossed your timeline, Doctor.
The Doctor looked at Ace. 'That explains the vortex stabiliser issues.'
Sarah frowned. 'Vortex stabiliser? Did you blow out the Gulag oscillator?'
The Doctor raised his eyebrows. 'How could you possibly know that?'
'I ran into the same thing with a Frunchelian freighter about six months ago. I helped them repair it, but in case I ran into it again, I picked up another one the next time a supply ship came through.' Sarah gave the Doctor a knowing smile. 'Besides, this isn't the first time you've had trouble with that, but you always told me you fixed it.'
Ace looked at Sarah. 'So you've got the part we need.' She turned to the Doctor. 'You were right, Professor, the TARDIS did bring us where we needed to be.'
'For more than one reason, I'm sure,' the Doctor said quietly.
Sarah smiled reassuringly at Ace. 'I'll get my tool kit and the oscillator and we'll get started on the repairs, shall we?'
The Doctor watched as Sarah grabbed a few items around her attic. While hugely pleased to see her, he could see the differences that only time and experience added. 'You didn't seem surprised by the TARDIS's landing,' he finally said.
Sarah looked up from her spot by the light table as she dropped a small device into her pocket. 'Well, she does have her favourite spot.' She walked up and stood in front of the Doctor. 'I guess I should've asked. Do you want my help, Doctor?'
The Doctor smiled easily. 'The chance to work on the TARDIS with my best friend? I certainly can't turn that down.' He extended an arm to her and the three of them entered the TARDIS to begin the repairs.