Sarah Jane Smith:

Sarah Jane first meets the Third Doctor when she manages to infiltrate a top secret research facility by posing as her aunt, Lavinia Smith, a famous virologist. Sneaking into the TARDIS while the Doctor is preparing to follow the trail of a kidnapped scientist through time, she becomes embroiled in the subsequent adventure in the Middle Ages. Sarah Jane then finds herself working with the Third Doctor and U.N.I.T. on a number of occasions. She is present when he regenerates into the Fourth Doctor and continues to accompany him on his journeys through time and space.

As a companion she is confident and inquisitive, and possesses a sharp mind as well as a sharp tongue. She is also a feminist and is infuriated when the Doctor asks her to make coffee. She often verbally spars with fellow companion Harry Sullivan, who has an old-fashioned and unintentionally patronising attitude towards her. However, this does not get in the way of forming a close friendship with him. During a visit to Peladon, the Doctor advises Sarah Jane to share concepts of Women's Lib with Queen Thalira, whose power is limited on her planet because of her gender. In later adventures, Sarah's views become less vocalised, but she never gives the impression that she is less than capable. In spite of the dangerous and frightening situations she often finds herself in, she loves adventure and risk. In spite of her outward complaints, Sarah Jane is always thrilled to go off in search of more adventure and shares a genuine rapport with the Third and Fourth Doctors. During her time with the Doctor, Sarah Jane encounters Daleks, Cybermen, antimatter creatures on the most distant planet in the universe, android mummies in 1911 England, ancient evils in 15th century Italy and other dangers.

Her final decision to leave the TARDIS and return home coincides with the Doctor's summons to his home planet, Gallifrey, to which he was forbidden to take humans at that point. However, his attempt to materialise the TARDIS near her flat in South Croydon falls short of the mark. At some point after this, the Fourth Doctor sends her a K-9 Mark III as a gift.  Some time later, Sarah Jane was herself summoned to Gallifrey where she was reunited with the Third Doctor in the Death Zone, and met the First, Second, and Fifth Doctors.

Thirty years later, she encounters the Tenth Doctor, Rose, and Mickey while investigating strange happenings at a school, and she reveals that he had in fact abandoned her in Aberdeen, Scotland. She tells the Tenth Doctor that, in the period after she was returned to Earth, she gave up waiting for him to return and concluded that he had died on whatever mission he had been sent. At the end of this adventure, he leaves her a new K-9 Mark IV, to replace the K-9 Mark III that sacrifices himself so that they may all escape from the Krillitane. In the time since her encounter with the Tenth Doctor,  K-9 has left Sarah Jane to close off a black hole but occasionally passes close enough to contact her.

Sarah Jane drives an emerald green Nissan Figaro and investigates alien interactions with Earth aided by a sentient supercomputer named Mr. Smith. She has a watch that scans for alien life and a lipstick-shaped

sonic screwdriver that were gifts from the Doctor, hidden inside K-9 Mark IV when he was given to her by the Tenth Doctor. Her low-key methods of investigation have helped protect the Earth from alien attack, which contrast to the more abrasive and militaristic approaches of U.N.I.T. and Torchwood , who, according to Sarah Jane, tend to go in "all guns blazing". Her investigations are covert and remain unknown to her neighbours in Bannerman Road, Ealing which has encouraged her reputation for being formidable and reclusive.

She adopted a son, Luke Smith, and befriended neighbour Maria Jackson during her investigation of the Bubbleshock manufacturing plant, remarking that since meeting them she is no longer content to live alone. Over time, she learns how to be a mother to Luke and starts to become more "mumsy" as she puts it. She and Maria share a close friendship to the point where Maria is the person Sarah Jane "trusts the most". Along with Luke and his friend Clyde Langer they defeat various threats to Earth such as the Bane, the Slitheen, the Gorgon and General Kudlak. When an alien entity called "the Trickster", so named by Alan Jackson (Maria's father), unsuccessfully attempts to remove Sarah Jane from Earth's timeline, she reveals that she witnessed the death of her friend Andrea Yates who fell from a pier they were both playing upon and who Sarah Jane was unable to rescue. It was this event that gave Sarah Jane a thirst for justice, and a strong desire to fight against loss of life.