Within the narrative of the series, Luke is a human archetype, created by alien species the Bane from thousands of DNA samples, who comes to be adopted by the series' heroine, Sarah Jane Smith. Luke is something of a child prodigy, showcasing a remarkable degree of genius but also a level of social ineptitude owing to being born in adolescence.
Luke Smith is introduced as the Archetype in the first episode of The Sarah Jane Adventures, the New Year's Day special "Invasion of the Bane". The Bane Mrs Wormwood creates the Archetype from thousands of samples of DNA taken from visitors touring their Bubble Shock! drink factory. This enables the Bane to run tests on an archetypal human. Luke appears to be an ordinary adolescent boy, except for his lack of a navel having been grown rather than gestated and born naturally. He possesses a superhuman intelligence and an exceptional eidetic memory.
During one tour of the factory, an alarm is set off when visitor Kelsey Hooper's mobile phone disturbs the Bane Mother. The Archetype awakes and proceeds to escape with visitor Maria Jackson and her investigative journalist neighbour, Sarah Jane Smith. After the Bane Mother sends assassins to kill the group, they return to confront the Bane. The Archetype saves the day by recalling an exceptionally long sequence of numbers necessary to trigger an explosion in the Bubble Shock! factory. In the episode's dénouement, Sarah Jane adopts the boy, naming him "Luke Smith", having also considered the names "Harry" and "Alistair" after her friends Harry Sullivan and Brigadier Alistair Gordon Lethbridge-Stewart. Her sentient supercomputer Mr Smith forges and distributes all the necessary paperwork to formalise the adoption. Sarah Jane reveals to Luke and Maria that she is a former time traveller, having been a companion of the Doctor and lives a dangerous life investigating alien invasions and schemes on Earth.
The first story of Series 1 of The Sarah Jane Adventures, Revenge of the Slitheen, depicts Luke's first day at school and his difficulties as to whether or not to refer to Sarah Jane as his "Mum". Luke struggles to adapt to life at school owing to his inability to lie or master humour and his apparent genius-level intellect which sets him apart from his fellow pupils. He and Maria (who is also new to the school having moved recently to the area) meet and befriend Clyde Langer, whilst Luke makes an enemy of Carl, an intelligent boy who envies his effortless intellectual superiority. The friends discover that Carl and three members of staff at the school have been replaced by members of the alien Slitheen family from Raxacoricofallapatorius, and that Luke has assisted unknowingly their plan to switch off the Sun by providing them with the equation they needed to run the machine that will absorb its energy. Luke tricks the Slitheen into resetting their machinery and most of the Slitheen are trapped in their secret room in the school and explode.
In Eye of the Gorgon, Luke is entrusted with a talisman by Bea Nelson-Stanley. The talisman is alien in origin and is the key to a portal in space and time sought by a Gorgon. Learning that Bea gave Luke the talisman, nuns controlled by the Gorgon kidnap Luke. Eventually, Luke is freed after Maria turns a mirror on the Gorgon causing it to fossilise itself. Warriors of Kudlak sees Luke and Clyde noted for their exceptional combat ability when playing laser-tag leading them being kidnapped by the alien Uvodni who seek to make them fight in a distant intergalactic war. Sarah Jane and Maria manage to teleport aboard the Uvodni spaceship and attempt to rescue the boys, but it is Luke who exposes the fact that the Uvodni's war has been over for a decade prompting the Uvodni to free their captives. Rewarding him for his heroism, one of the rescued children, Jen gives Luke his first kiss.
In the finale of Series 1, The Lost Boy, a couple alleges that Luke is their biological son. Mr Smith verifies the accuracy of their evidence and purports that perhaps Luke was abducted and altered by the Bane rather than grown by them. Luke is returned to his "parents" and is told his real name is "Ashley"; Sarah Jane is pronounced a child abductor but is released from prison by UNIT. Luke's "parents" are abusive, later revealing themselves as Slitheen in league with the mysterious "Xylok" - in turn later revealed to be Mr Smith's true identity. Mr Smith recruited the Slitheen to capture Luke for the purpose of using his latent telekinetic ability to destroy the Earth and release his Xylok brethren. The child Slitheen, having survived the explosion in Revenge of the Slitheen, has developed a device that harnesses telekinetic power which Mr Smith has Sarah Jane steal under the pretence of analysing it and uses it in conjunction with Luke to bring the Moon crashing towards Earth. Discovering that Mr Smith is the evil Xylok and that he is betraying the Slitheen, Sarah Jane works with the Slitheen and her robot dog, K-9 Mark IV to defeat the Xylok, using computer virus to give him a new benevolent raison d'être. Luke is reunited with Sarah Jane as the Slitheen return to Raxacoricofallapatorius.
Luke appears in the finale of Series 4 of Doctor Who, alongside Sarah Jane, Mr Smith and K-9, witnessing a Dalek invasion of Earth. With Earth having been transported across space for use in the Daleks' "Reality Bomb", Luke assists the Doctor and his associates in using the TARDIS in conjunction with Mr Smith and the Cardiff Rift to tow Earth back to its rightful place. Following this, series two of Sarah Jane sees Luke part ways with Maria when she moves to America in the The Last Sontaran, and befriend investigative new girl Rani Chandra who comes to discover the extraterrestrial activities that he, Clyde and his mother are involved with. In Doctor Who episode The End of Time, Luke is saved from being run over by a car while talking on his mobile phone to Clyde by the Doctor just before his pending regeneration. The Doctor then waves to Sarah Jane and Luke before stepping into his Tardis and disappearing.
In the first and second series, Luke and Maria spent a lot of time together. It is possible that they like each other, although Luke insists that they are just friends when Clyde interrogates him about it.
In The Sarah Jane Adventures Series 1 story Whatever Happened to Sarah Jane?, the timeline is changed by the malevolent Trickster such that Sarah Jane dies as a child. Consequently, Luke ceases to exist in this reality and no one remembers Sarah Jane, Luke or Mr Smith apart from Maria, protected by an alien puzzle box, who successfully conspires to return the timeline to normal.
Later, Luke is mentioned in Doctor Who Series 4 episode "Turn Left", in which Donna Noble is assaulted by a member of the "Trickster's Brigade" and consequently has an alternative universe created around her. In this reality, Donna never meets the Doctor, resulting in his death and Luke is said to have been killed along with Sarah Jane, Maria, Clyde and medical student Martha Jones stopping the events of Doctor Who Series 3 episode "Smith and Jones" in place of the Doctor. The real timeline is eventually restored by Donna due to the intervention of Rose Tyler, a traveller from a parallel universe.