One Degree Of Separation / Whoniverse

• Through her association with the Doctor, Sarah Jane Smith also became a long-time friend of Brigadier Lethbridge-Stewart, who met Fifth Doctor companion Tegan Jovanka in "Mawdryn Undead". Later, when circumstances bring the three into the same room, the two ladies call out the Brigadier simultaneously.
• Doctor Who Magazine's Brief Encounters series of short stories is based on this trope. One example, "Ships" has former First Doctor companion Dodo Chaplet bump into Sarah Jane Smith, who travelled with the Third then Fourth Doctors.
• The Big Finish Doctor Who short story collection The Centenarian. It's not just that the titular Centenarian, Edward Grainger, keeps running into the Doctor. It's not even that the Sixth Doctor also met Edward's granddaughter in a previous short story collection. It's that Edward's mother just happens to be a good friend of Lady Louisa Pollard — mother of Eighth Doctor companion Charley Pollard.
• "Aliens of London" has the Ninth Doctor meeting a Dr. Toshiko Sato. Unbeknownst to him, she has already been recruited to the Cardiff branch of a certain Institute by one Captain Jack Harkness, whose younger, pre-immortal self he will meet relatively soon in World War II. She is still there when the TARDIS pops in Cardiff and inadvertently picks Jack up, one of Jack's "little gang" who were sent on a "wild goose chase" by the Master to prevent them from assisting the TARDIS team against his plot, then Jack is returned home. Sadly, by the time circumstances bring the Tenth Doctor in contact with Jack and team, Tosh has been killed in action.
• One Christmas sees circumstances bring the Tenth Doctor into an adventure with "runaway bride" Donna Noble, involving disaster at the wedding reception. The next Christmas ses the Doctor meet a newspaper vendor named Wilfred Mott. As it turned out, Wilf is Donna's granddad and is most pleased to recognise, through the telescope, the suited man standing next to Donna as she waves to him from the "blue box" she was so obsessed with. It isn't until Donna takes a stop back home, that Wilf points the Doctor out; Wilf missed Donna's wedding having been laid up with Spanish flu. It should be noted that this connection is only due to an unexpected death; Wilf (formerly Stan) was originally a one-off character before taking the place of Donna's father from "The Runaway Bride", whose actor passed away during the early filming of Series 4.
• Exploited in "The End of Time", where the Tenth Doctor is aware that a post-Children of Earth Jack is drowning his sorrows at the same space bar as Alonso Frame, who previously faced disaster with the Doctor, and so arranges for them to hook up, the story of which is eventually told in the audio story "One Enchanted Evening".
• Big Finish's The Diary of River Song has River bumping into not just the classic incarnations of the Doctor, but several incarnations of the Master and several of the Doctor's old friends.
• The Torchwood audio story "Sync" is an interquel which sees the meeting of two murderous women in Cardiff: Torchwood member Suzie Costello (notable for going rogue and dying for the first time in the first episode) and a certain flatulent alien disguised as Lord Mayor of Cardiff. After spending the episode surviving the technobabble which binds them together without killing each other, the pair both recognise that the other has skeletons in their closet and so make a mutual pact to stay out of each other's way. "Margaret Blaine" never realises that Suzie's boss is Jack Harkness, whose younger self was in the company of the Ninth Doctor when he catches up with her.
• The Class (2016) audio story "In Remembrance" has an older Ace, former companion of the Seventh Doctor, return to Coal Hill School to deal with a temporal anomaly and a Dalek. There she meets Andrea Quill who, along with Charlie Smith, is an alien who was rescued and relocated by the Twelfth Doctor, and it isn't long before the two realise they have the Doctor in common. Things get timey-wimey when Charlie is displaced to 1963, during the events of "Remembrance of the Daleks" and ends up meeting Ace in the past.
• A peculiar example occurs in the Lady Christina story "Skin Deep" , where the eponymous protagonist and acquiantance of the Tenth Doctor meets Sylvia Noble, mother of Donna. Christina later faces a Sontaran then the Slitheen with Sam Bishop of U.N.I.T., colleague of Kate Stewart, daughter of The Brigadier and friend of the Doctor.

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