Mr Smith

Mr. Smith was a super-computer (of sorts) owned by Sarah Jane Smith, located in the attic of Sarah Jane's house at 13 Bannerman Road it was shortly revealed that he was actualy a Xylok embeded into the computer.

Origins
The computer known as Mr Smith housed a sentient and highly intelligent crystal known as a Xylok. In the distant past a Xylok ship crashed into the Earth, trapping the Xyloks in the crust of the planet. When the volcano Krakatoa erupted, one crystal escaped. Centuries later, Sarah Jane Smith retrieved the living crystal, took it to her home on 13 Bannerman Road, and found it was able to communicate with her via her laptop computer. The crystal informed her how to build the computer which would house the living crystal. Sarah Jane did not know at this time about the Xyloks or that Mr Smith ultimately planned to free the other Xylok, even at the cost of the planet itself. (SJA: The Lost Boy)

Reprogramming
Mr Smith teamed up with the remaining Slitheen on Earth, and, using deception, helped two of them to pose as Luke Smith's true parents so that they could kidnap him. Alan Jackson used a computer virus against Mr Smith which disabled him and wiped his database. When he forgot his purpose Sarah Jane informed him of his mission to save humanity which he accepted and shut down. (SJA: The Lost Boy)

After Reprogramming
Mr Smith was of great assistance to Sarah Jane and Luke during the initial phases of the Dalek invasion of Earth in the 21st century. He alerted the Smith family to the Earth's changed position in space. Later, he identified the presence of incoming space ships, and relayed the message of that fleet to the Smiths. Once that fleet had been identified as Dalek in origin, he amplified an incoming sub-wave message from Harriet Jones. This latter message allowed a pivotal conference call between Sarah Jane, Martha Jones, Torchwood Three, and — unbeknownst to Sarah — Rose Tyler. Together they formulated a plan of action to contact the missing Tenth Doctor, in the hopes that he could help repel the Daleks. In this scheme, Mr. Smith vitally linked together all the world's phone networks to dial the number of Martha Jones' phone aboard the TARDIS. Mr. Smith's efforts were relayed to Torchwood, whose rift manipulator greatly amplified his signal. Due to the power needed to amplify Mr. Smith's phone call, the computer may have taken heavy damage in the attempt. Nevertheless, he was able thereafter to detect the presence of the Doctor's TARDIS landing, prompting Sarah Jane to leave Luke in Mr. Smith's care while she attempted to physically meet with the Time Lord.(DW: The Stolen Earth)

Later, Mr. Smith linked with the TARDIS and Torchwood Rift in order to allow the Doctor to tow the planet back to its proper place in space and time. (DW: Journey's End) By the second series of the Sarah Jane Adventures, his logo has changed to a more fluid graphic and he has apparently developed a sense of humour. (SJA: The Last Sontaran) Mr Smith told the team more about the Pied Piper including that he came from the Jeggorabax Cluster but he needed more information so asked Sarah Jane to go to the Pharos Institute to collect a part of the meteorite. (SJA: The Day of the Clown) When Martin Trueman was possessing the world to join the Ancient Lights even Mr Smith was getting controlled and was losing connection. (SJA: Secrets of the Stars) He was able to trap threats via the use of a containment vortex for example over Mrs Wormwood. (SJA: Enemy of the Bane) Through the Sonic Lipstick he teleported a Slitheen back to Raxacoricofallapatorius. (SJA: From Raxacoricofallapatorius With Love)

Mr Smith received a distress signal and video from a Judoon who was in a ship which was crashing to Earth. He told the gang where the ship was crashed. Later on Sarah Jane (possessed by Androvax) activated Mr Smith's self-destruct program, which would destroy all of Bannerman Road after 60 seconds. Luke reminded Mr Smith that purpose is the most important thing to a xylok and that his purpose was to protect the Earth,so the timer was stopped. (SJA: Prisoner of the Judoon) He helped Sarah Jane find Rani who was in Danemouth, and then put Sarah Jane's phone through to K-9 to use the black hole to power Eve's ship. This allowed K-9 to come back, with Mr Smith showing his dislike very quickly.(SJA: The Mad Woman in the Attic) Since then, Mr Smith and K-9 appear to have an antagonistic relationship with each other, attempting to one up each other in the presence of Sarah Jane or the children. (SJA: The Wedding of Sarah Jane Smith) He showed a report from the disappearance of the Mona Lisa as well as telling her that the painting which had Phyllis Trupp in was the actual painting.(SJA: Mona Lisa's Revenge) When the Slitheen-Blathereen gave Sarah Jane the Rakweed, she told Mr Smith to scan it finding out that it was harmless.However, the morning after, the weed mutates and turns harmful attacking Luke Smith and Sarah Jane but Mr. Smith uses his airvents and sucks the weeds away. (SJA: The Gift)

Frequently Used

 * He possessed an extensive, but not absolutely complete database of alien life forms and could display them on his screen. (SJA: Invasion of the Bane onwards).
 * He secretly monitored the activity and communications of alien ships.
 * Mr Smith played a fanfare of his own accord when activated (SJA: Invasion of the Bane onwards) as shown when Sarah Jane Smith requested that he stop playing it during his activation following the transportation of Earth to the Medusa Cascade. (DW: The Stolen Earth) Luke Smith had also requested Mr smith start in silent mode during Clyde, Rani, K-9 and his attempts to spy on Sarah Jane (SJA: The Wedding of Sarah Jane Smith)
 * Mr Smith could relay messages from alien sources and display them on his screen. (SJA: Invasion of the Bane, Warriors of Kudlak, Prisoner of the Judoon, DW:The Stolen Earth)

Less Commonly Used

 * He diverted meteorite K67 away from colliding with Earth, using magnetic repulsion. (SJA: Whatever Happened to Sarah Jane?)


 * Mr Smith could also readily detect celestial bodies and ships in close proximity to the Earth, such as the other 26 bodies and the Dalek fleet located in the Medusa Cascade. (DW: Journey's End)
 * He could tap into every phone network on Earth. (DW: The Stolen Earth)
 * The computer seemed to have heavy armaments, including projectile weapons. (SJA: The Lost Boy)
 * Mr Smith was also capable of entrapping alien threats within a containment vortex and sapping their weapons of their energy. (SJA: Enemy of the Bane)
 * Mr Smith also has a Self Destruct setting which was capable of destroying the whole of Bannerman Road. This was activated by the Veil Androvax when he possessed Sarah Jane's body, when Clyde,Rani,Luke and Captain Tybo found traces of Androvax in Sarah Jane's House, Mr Smith warned them to leave as soon as possible as the house would blow up in 60 seconds (SJA: Prisoner of the Judoon).
 * He could be used to contact K9 from inside the black hole. (SJA: The Mad Woman in the Attic).
 * He has an air ventilation system with large fans hidden under his computer screen (SJA: The Gift).

Personality
Initially, Mr. Smith carried himself with a cold, clinical mindset, more like an actual computer then a living being. When his true intentions were revealed, he started acting malicious and cruel, yet at the same time he would justify his actions by stating that they were part of his "purpose."

After his memory had been wiped by the Armageddon Virus, Mr. Smith's personality changed greatly. He became much friendlier and more amiable, to the point where he would make jokes. He even bid Maria a fond farewell when she left for America. (SJA: The Last Sontaran)

When K9 returned to Bannerman Road, Mr. Smith entered into a deep rivalry with him, the two constantly trying to one-up each other in the technical support area, and referring to each other as "the dog" and "that computer".

Behind the Scenes

 * Mr Smith's name is a matter of some speculation. He could be named for "John Smith", the often-used alias of the Doctor. Alternately, Sarah could have simply used her own last name to christen the computer, or the name may have come from the phrase written on the back of a photo of her parents "Mr. Smith, I need you" as shown in The Temptation of Sarah Jane Smith.
 * It is not until Sarah Jane complains about it in The Stolen Earth that Mr. Smith's fanfare is actually acknowledged as being something the characters hear, rather than incidental music for the viewer's benefit. The fanfare is possibly a reference/parody to the Windows start-up music.
 * A possible reference to Mr Smith appears in The Bounty Hunter. In a news report it shows that a Mr Smith won the lottery and also asking anyone who is this man.