After travelling with The Doctor in four seasons of the show they suddenly part ways, and after this she continues to investigate strange goings-on back on Earth.

Over time, Sarah Jane establishes herself as a committed defender of Earth from alien invasions and other threats, occasionally reuniting with The Doctor in the course of her own adventures, all the while continuing to work as a freelance investigative journalist.

[1] Doctor Who producer Barry Letts offered the part of Sarah Jane to actress April Walker, who accepted the role and was duly contracted by the BBC.

[3] Letts began a second batch of auditions and saw Elisabeth Sladen after a recommendation from fellow BBC producer Bill Slater, who had twice cast the actress recently in separate episodes of Z-Cars.

Sladen performed her audition alongside actor Stephen Thorne and after impressing Letts, he arranged for her to meet Pertwee before any decisions were made.

[4][5] 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, and she also assists the United Nations Intelligence Taskforce led by Brigadier Lethbridge-Stewart (Nicholas Courtney) on a number of occasions.

Sarah Jane departs in the Season 14 serial The Hand of Fear (1976) after The Doctor receives a summons to his home planet, Gallifrey.

In the pilot episode, titled "A Girl's Best Friend", Sarah Jane receives the robot dog K9 Mark III (John Leeson) as a Christmas present from The Doctor.

The character made an appearance in the 1993 Children in Need special (a crossover with long-running British soap EastEnders), Dimensions in Time, wherein various Doctors and companions are teleported to Albert Square as part of a plot by the Rani (Kate O'Mara).

Though Sladen would not make any more official Doctor Who television appearances until 2006, in the interval the actress reprised her role as Sarah Jane twice on BBC radio with Jon Pertwee, and in a series of Big Finish audio dramas, and in the unofficial direct-to-video spin-off film Downtime (1995) alongside the Brigadier, Second Doctor companion Victoria Waterfield (Deborah Watling) and Leeson in a rare on-screen role.

In The Sarah Jane Adventures, Sarah Jane investigates alien activity covertly from her manor house in Bannerman Road in Ealing, driving an emerald green Nissan Figaro[7] and with the help of her sentient supercomputer Mr Smith (voice of comedian Alexander Armstrong), as well as an alien activity scanner and sonic lipstick device.

Sarah Jane remarks that since meeting them she is no longer content to live alone; she discloses she never married after parting from the Fourth Doctor, to whom no one could ever compare.

Amongst these threats, series one introduces The Trickster (Paul Marc Davis), a cosmic being who makes alterations to the timeline to cause chaos and destruction; he becomes a recurring adversary for Sarah Jane.

In series three (2009), Sarah Jane makes a new enemy in the body-stealing alien Androvax (Mark Goldthorp) as well as a lukewarm ally in the form of the interplanetary police force, the Judoon (from Doctor Who).

In series finale The Gift, though she is reluctant to use weapons or cause harm, when Luke falls deathly ill for the first time ever due to the machinations of the Blathereen-Slitheen, Sarah Jane takes up arms to confront them.

When The Doctor is slowly dying from radiation poisoning, he makes timely visits to his close friends and companions; The Doctor saves Luke's life from an oncoming car on Bannerman Road (an inside joke by Russell T Davies commenting on actors' failure to look for cars before crossing the street because traffic is always stopped for them, something that he believes is especially important in a children’s show) and bids a silent farewell to Luke and to Sarah Jane.

[12] The Empty Planet is this series' 'Sarah-lite' serial; with the rest of the population, Sarah Jane mysteriously vanishes, leaving Rani and Clyde to investigate without her.

The final story ends with a short montage of archive footage and audio recordings celebrating Sarah Jane's journey from lone investigator to mother of two surrounded by friends.

In the Tales of the TARDIS spin-off episode "Pyramids of Mars", the Fifteenth Doctor tells his companion Ruby Sunday (Millie Gibson) about Sarah Jane and that they were best friends and told her that she travelled with him during his first encounter with Sutekh.

It revealed that Sarah Jane was working as UNIT's official chronicler and that in "1996" she covered the aftermath of the Cybermen's failed attempt to drain Earth of its energy and the technology left behind in their wake (The Tenth Planet, which was actually set in 1986).

She wrote a history of UNIT, Fighting for Humankind ("heavily vetted and evidently subjected to considerable censorship"), as well as a series of science fiction novels.

These featured an extraterrestrial called The Doctor and his companion Nicola Jones, who frequently encountered WIN (World Investigative Network), commanded by General Lutwidge-Douglas.

Afterwards she developed a romantic relationship with Harry Sullivan; they parted amicably and were still very fond of each other, and were still close friends, even to the time of the article.

Between seasons 13 and 14, Sladen appeared as Sarah Jane, with Tom Baker as the Fourth Doctor, in the audio play The Pescatons, released on LP by The Decca Record Company Limited in 1976.

[21] The canonicity of Sarah Jane's appearances in the audio dramas is, like all Doctor Who spin-off media, unclear, and they may not even take place in the same continuity as one another.

[31] The Daily Telegraph's Gavin Fuller also ranked Sarah Jane number one, praising Sladen's portrayal and saying that she displayed "great determination and bravery".