So I've started watching some of Old Who on Netflix, and I think Netflix may not have all the "classic" episodes (as they put it) on there - it seems to skip over some of the specials, at least. Anyway, the way the series is shown, Sarah Jane seems to very suddenly/randomly appear and then disappear as the Doctor's companion during Tom Baker's stint as the Doctor. I think she may come back later based on some episodes I've seen out of order, but I was just wondering whether anybody knew how the Doctor and Sarah Jane met and why she (at least temporarily) disappeared.
From Wiki, Sarah Jane first appears in the episode The time Warrior (1973): she hides in the TARDIS after she met the Doctor in a top secret facility.
From Sarah Jane's Wiki:
Sarah Jane first appears in the Third Doctor serial The Time Warrior (1973-74), where she has managed to infiltrate a top secret research facility by posing as her aunt, Lavinia Smith, a famous virologist. Introduced as an ardent feminist, Sarah Jane sneaks aboard the TARDIS and becomes embroiled in a battle against a militaristic alien Sontaran in the Middle Ages who is kidnapping scientists from the present day. Subsequently, she accompanies the Doctor (Jon Pertwee) on several journeys in the TARDIS
She leaves after about two years when the Doctor has to go back to his planet.
She meets him again in her own series and also in the reboot of Doctor Who.
By the way it seems that many episodes of the classical Doctor Who series are now lost: see here