A bit more on Sarah Jane Smith and K-9, in preparation for "School Reunion".
Sarah Jane meets the Doctor (Page from Shillpages)
Sarah Jane first met the Doctor in his Third incarnation when she was an undercover reporter in "The Time Warrior". She found herself unexpectedly transported to medieval times, and initially suspected the Doctor was behind the mischeif. However, she grew to trust him and joined him in the fight against the Sontaran Linx and his adversaries.
Eventually upon the return to present-day Earth, she started to hang around with UNIT and the Doctor. With the third Doctor she fought Dinosaurs, Daleks, and Ice Warriors. On an adventure on Metebellis Three, the Doctor was forced to regenerate and became the fourth Doctor.
The Third Doctor regenerates
With the Fourth Doctor, Sarah Jane's trips in the TARDIS became far more frequent (She also travelled for a time with UNIT doctor Harry Sullivan). She encountered Sontarans and Daleks again, and met enemies such as the Wirrn, Cybermen, Sutekh, Krynoids, and the Zygons.
Sarah Jane and the Fourth Doctor
Eventually, after being posessed by the being Eldrad (In the Hand Of Fear) Sarah Jane had a minor tantrum and packed her bags-but she didn't mean it. However, the Doctor received a summons from Gallifrey, and Sarah couldn't come along...so he had to leave her behind. Unfortunately, he left her in the wrong place.
However, the Doctor didn't forget Sarah Jane. He built her a new K-9 unit, based on the Doctor's previous two models which served as companions. The K-9 helped her solve a mystery involving a cult.
A few years later, Sarah Jane, despite K-9's warning, was sucked into a time scoop and deposited into the Death zone on Gallifrey, where she teamed up with the Third Doctor once again. She also met the Master for the first time during this adventure and once again confronted the Cybermen. She also met the Doctor's other incarnations, but was somewhat confused and didn't really have a proper reunion with the Fifth Doctor. She later was returned to her own time.
In the years that followed, K-9 fell into disrepair and Sarah Jane continued her investigations. A few WHO books and audios continue her adventures, however their canonicity remains in dispute (Especially her adventures with the Seventh Doctor in the DWM comic strips and Bullet time).
It's been thirty years since the Doctor dropped her off, and she's about to have a unique "School reunion"...