Sarah Jane Smith was a journalist who travelled with the Doctor in his third and fourth incarnations and was also acquainted with his tenth and eleventh. Together with her friends she met and battled aliens that came to Earth.
At age 23, when working for Metropolitan magazine, Sarah posed as her aunt, a famous virologist, in order to infiltrate a UNIT-controlled facility where some scientists had gone missing. She met Brigadier Lethbridge-Stewart and UNIT's unpaid scientific advisor, the Third Doctor. Suspicious of the Doctor's activities, Sarah Jane stowed away in the Doctor's TARDIS and traveled back in time with him to 13th century England, to defeat a Sontaran known as Linx, who had kidnapped the scientists.
Along with another human companion, Harry Sullivan, Sarah Jane and the Doctor traveled off on other adventures in time and space. The Doctor, in his new incarnation, had less interest in Earth, yet he seemed to have a closer relationship with Sarah Jane herself. She was present on the planet Skaro during the creation of the Daleks by Davros and the Doctor's attempt to change or prevent it.
Finally, after many trips with him, the Doctor received a summons to return to Gallifrey, his home world; as a human, she could not accompany him. The Doctor attempted to deposit her near her flat in South Croydon, but mistakenly left her in Aberdeen, Scotland, instead.
As a companion, Sarah Jane was confident, inquisitive and possessed a sharp mind and a sharp tongue. She was also something of a feminist; she was infuriated when the Third Doctor asked her to make coffee and she often verbally sparred with fellow companion Harry Sullivan over his chauvinistic and unintentionally patronising attitude towards her. Her feminism was more practical than fanatical, and did not get in the way of forming close friendships with Harry, or later with the Doctor himself. She shared a strong rapport with the Doctor, and her subsequent life has been shaped by her finding him a tough act to follow; she stops short of saying that she had fallen in love with the Doctor, though several of her exchanges with him after their reunion, in addition to the context of one of her conversations with Rose, strongly implies that she indeed had developed feelings for him. Despite the passage of time she maintained her can-do personality, and since reuniting with him, remained a close friend of the Doctor.
Rani once pointed out to the Tenth Doctor that Sarah Jane doesn't like being called just "Sarah", though he replied that "she does by me." Indeed, during her years traveling in the TARDIS the Doctor, along with a few other individuals such as Harry, and even Mike Yates, referred to her numerous times simply as "Sarah". To date, the Doctor remains the only person to still regularly call her this without her objection.
Sarah received training in evasion tactics from her UNIT days, which she never forgot, she stated "U.N.I.T training, never forgot it, just getting a bit older". She gained superb leadership skills from her time with the Doctor and was very good with her Sonic lipstick. She was intelligent and very knowledgeable about many types of aliens.
