I do not own any part of Doctor Who. It is the Sole Property of BBC and the British. This is just a simple footnote event in the history of the Doctor.
This takes place in the time of the third Doctor, several months before he takes the crystal back to the spider.
"Ah, there you are Miss Smith; I was just coming to look for you."
"Oh really Doctor, and why is that?" asked Sarah Jane Smith.
"Just that something has come up and I thought you might want to come along."
"What is it now, more daleks, or maybe Cauntarins,. No wait, The Master again."
"Oh nothing so mundane," said the Doctor. "I just received a most interesting invitation by time displacement mail."
"Time displacement mail?"
"Yes, a mail service in the 65th century, they deliver mail to any time and any world. Cost a bundle to send even a simple thing like this." He held up a simple tan colored envelope for her to see. "It is addressed to me in the Tardis, and I can bring one guest."
"And just what is it you have been invited to might I ask?" Her hands on her hips, giving him the look that said he was going to get in trouble again.
"Oh nothing major. It is just a lecture and presentation of the results of an archeology expedition. It apparently took several decades and decoded the mystery of a burned out world."
"Nothing like dusty old professors going on for hours to make just one point," said Sara in one of her more sarcastic tones.
"Now, now Sarah, no need to be that way about it, the lead speaker is a woman, so that might appeal to you."
"Well in that case, let us be on our way, if you think you can get the Tardis to actually take us where you want the go for once."
"The letter does include very precise space/time coordinates, and written in High Gallifrain at that. So come along, grab your coat and we'll be off."
They boarded the Tardis and with its normal whine and groaning of a worn out dimensional stabilizer, vanished from sight.
Just as the Tardis finished dematerializing, the Brigadier entered the lab, "Doctor, I have a ... Gone again. I wonder for how long this time." He turned and returned to his own office.
"Well Sarah, it looks like we are going just where I set the controls to go."
"That is a nice change. Last time you said that, we ended up dealing with a creature in the tunnels under the castle as you may recall."
"Yes well, that all on Peladan turned out the best now, wouldn't you agree?"
"If you say so Doctor." She circled around the console watching the energy readings on the time line monitor. She noticed it had started to rise in a way she had never seen before. "Doctor, come look at this, the line in in the red instead of the green."
The Doctor came around and looked at what she was pointing at. "That is most odd. The only time that happens is if I come close to crossing my own timeline. I don't ever recall being here before though."
Sara looked at him with slight annoyance. "Well maybe you come here in the future and are their now."
His half smile told her she was right on the money with her guess as to the cause. "Well then, we just have to be more careful than normal then. If you see another Tardis or me, turn and walk away quickly, in fact come right back here, to this Tardis."
"Alright Doctor, how will I know it is you if you have changed again."
"Hmm, good point."
The console pillar stopped moving and the arrival bell sounded. "Now we go out and see the sights." He grabbed his cloak, threw the lever for the door, and stepped outside. He stopped as he had a number of rifle barrels pointed at his head.
The guard in charge told him, "Don't move until we say you can can move, or your head comes off, regeneration or no."
The Doctor said, "What is the meaning of this, I was invited here for the conference on the dig that Professor Song led."
Another guard spoke up, "Wrong time zone, this is a much earlier Doctor, he has no knowledge of the events. He is clear. You companion may come out now as well."
"You can come out now Sarah." Sarah poked her head out in time to see the rifles being shouldered.
"I take it that the Doctor will do something in the future that makes him unwelcome here?"
"Not the Doctor, no. One of his travel companions. But I will say no more. You don't need to know for your own safety."
"If it means I am at risk of crossing my own timeline, then I think I will stay and find out why I received this invitation. It may have been meant for me, but not me."
"Not our worry Doctor, the hall is over by that tower, enjoy your stay at colony 5145 Doctor."
He turned and followed his departing troops.
"Now that was disturbing, wouldn't you say Sarah?"
"Just a little. Do you think it is safe to stay, I mean, really safe?"
"As long as we leave right after the conference we should be alright. Come along now."
.... There is more of this story ...