Tenth Doctor Month: Meet The Companions Sarah Jane Smith

Sarah Jane Smith was a journalist who claimed to be writing a profile for the Sunday Times of Hector Finch and the amazing performance of Deffry Vale High School's students since his arrival as Headmaster. In truth she was investigating for the same reasons as the Doctor, Rose Tyler and Mickey Smith - UFO sightings three months previously.
Some years before she had travelled with the Doctor, eventually leaving him abruptly when he had been summoned back to his home planet. He later left her a gift of K-9 Mark III, but that had not been enough for Sarah, who had found it difficult to go back to normal life without him. After initial hostility between her and Rose, the two became friends, although Sarah tried to warn Rose of the dangers of believing that she would travel with the Doctor forever (School Reunion).
In the alternative world where Donna Noble had never met the Doctor, Sarah Jane, along with her adopted son Luke and his friends Maria and Clyde went to the moon in the Royal Hope Hospital to face the Judoon and all four of them died (Turn Left).

When the Daleks invaded Earth to take people away for their Reality Bomb experiments, Sarah left Luke with Mr Smith and K-9 while she, Jackie Tyler and Mickey Smith were transported to the Crucible. There she met Captain Jack, Martha Jones and Donna Noble and was reunited with the Doctor and Rose. Another shock was in store when she met up again with the Daleks creator Davros, who thought it was ironic that they had met both at the beginning and the culmination of his campaigns. After the Daleks and Davros were defeated, Sarah went home to Luke (The Stolen Earth / Journey's End.)
Alerted by Luke one afternoon, Sarah saw the Doctor on Bannerman Road and realised he had come to say goodbye because he was going to regenerate soon (The End Of Time Part II).