This was the same year the Tenth Doctor was reuinited with Sarah Jane Smith, 2006--the television episode, "School Reunion", airing in April while The Gathering aired in September. It's somehow not often the Doctor encounters an old companion again outside of an anniversary episode. He ran into the Brigadier sporadically throughout the 80s but, hey, that's just the nature of the Brigadier. But unlike "School Reunion", The Gathering has a companion meeting up with the same Doctor she left, only it's twenty years in the future for the companion and presumably a shorter period of time for the Doctor. Tegan is an interesting companion to revisit, too, because she's the only one to have left because of the fear and anxiety brought on by travelling with the Doctor--something you'd actually think would happen more often. I actually would have preferred to have left that parting as it was. Sometimes people don't get closure.
But, oh well, like I said, The Gathering is good. Set in Tegan's native Australia--Brisbane--the Fifth Doctor (Peter Davison) finds her leading a friendless existence in an office job where the only people who show up at her birthday party are coworkers. The anxiety that drove her from the Doctor is still dominating her life.
But there's a lot more going on than that. Of course, I do know the reason the Doctor never meets himself--the TARDIS prevents him from landing in too close proximity. Nevertheless, the Doctor appearing on Earth for decades ought to run into things about him that may be in the distant past or distant future. That's just what happens here--Joseph Lidster, who wrote the episode I talked about last week, The Reaping, also wrote The Gathering and he brings back a minor character, Katherine Chambers (Jane Perry) who remembers her encounter with the Sixth Doctor twenty years earlier. Now the Doctor has to worry about her giving him too much information about his future.
It gets complicated. Katherine is a friend of Peri's family and Peri is at the time of this audio play a companion of the Fifth Doctor. But she doesn't appear because, as he explains off-handedly, he left her and the audio-only companion Erimem in Monte Carlo. Katherine is also an acquaintance of Tegan's but their meeting had nothing to do with the Doctor. Which is a coincidence but given that the Doctor has been appearing on Earth all the time for decades and will continue to do so for decades, odds are, if he doesn't meet people he's already met--even if he isn't at the point yet where he met them--he should at least meet plenty of people who have met or are related to people he's met.
This is actually more of a problem for the modern Doctors than the classic series Doctors. The classic series Doctors aren't shown visiting contemporary Earth nearly as often. Even the Third Doctor, when he was exiled to Earth, was at least exiled to just one time and place. How do we square the Ninth through Twelfth Doctors visiting 2005 to 2015 in roughly chronological order? Eleven says a period in New York in the 30s is blocked for him; it's a wonder he can visit the period between 2005 and 2020 at all, what with all the him roaming about. Considering he just leaves Amy and Rory and Clara on Earth, and the TARDIS is notorious for putting him a week or two ahead or behind where he intended, shouldn't he have occasionally had adventures with Claras or Amys before he had earlier adventures with them? Wouldn't it had happened sometimes without either party even realising it?