Mr Smith

Mr Smith was a super-computer owned by Sarah Jane Smith, located in the attic of Sarah Jane's house at 13 Bannerman Road. It was revealed that he was actually a Xylok embedded 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. It is believed that that earth shock wiped out the dinosaurs, but it may be speculated that actually it was the Xyloks that did. 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. 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 Xyloks, even at the cost of the planet itself. (SJA: The Lost Boy)

Working for Sarah Jane
After she created the computer to house him, Mr Smith began to help Sarah Jane, and gave her knowledge on many things. His help at this point however, was mainly confined to hacking computers and displaying information on alien life forms. He did reveal his dark side momentarily to Clyde Langer, presumably because he realised there was a possibility of getting help from the Slitheen. After this Alan Jackson gave him a computer virus and then Sarah Jane gave him a new purpose. (SJA: Invasion of the Bane, The Lost Boy)

Reprogramming
Eventually, Mr Smith was able to team up with a number of the Slitheen family who were on Earth. He 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 helped to save the Earth. (SJA: The Lost Boy)

After reprogramming and assisting the Bannerman Road gang
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 Jane, Rose Tyler. Together they formulated a plan of action to contact the missing Doctor, in the hope 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 took some 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 received the TARDIS base numerals from K9 and linked with the TARDIS and Torchwood Rift, allowing the Doctor to tow the planet back to its proper place in space and time. (DW: Journey's End)

His on screen logo changed to a more fluid graphic and he 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 became 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) Using 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 was the most important thing to a Xylok and that his purpose was to protect the Earth, causing Mr Smith's actions to be against his purpose and therefore stopped the countdown. (SJA: Prisoner of the Judoon)

Feud with K9
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 appeared to have had 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 fof the disappearance of the Mona Lisa as well as telling Sarah Jane 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, and he found it harmless. However, the morning after, the weed mutated and turned harmful, attacking Luke. When the spores threatened him and Sarah Jane, Mr Smith used his cooling air vents to suck the weeds. Mr Smith monitored Luke's condition during the crisis. When Clyde, Rani and K9 discovered how to stop the Rakweed, K9 sent Mr Smith the frequency level required to spread the alarm bell sound across London (1421.09 Hertz). On Sarah Jane's order, he did this again when the two Blathereen attacked, causing the Rakweed in their stomachs to be destroyed, creating a build-up of methane gas which caused them to explode all over everyone, including Mr Smith. Clyde then had to clean him up as a punishment by Sarah Jane for using K9 to cheat on his test. (SJA: The Gift)

When the Nightmare Man put Luke into a deep sleep, Mr Smith used a USB cable to give K9 extra power in order to contact Luke. He was attacked and damaged by the Nightmare Man, but later recovered. Before K9 left, Mr Smith told him that he could contact him if needed, which K9 took to mean Mr Smith would miss him. (SJA: The Nightmare Man)

More adventures
When Androvax came back, Mr Smith trapped him in the containment vortex. (SJA: The Vault of Secrets) When the Automatons removed all the humans from Earth bar Clyde and Rani to look for the heir to the throne of their planet, Mr Smith was taken as well, leaving the computer an empty shell as Rani discovered. He was then returned when the Automatons found the heir. (SJA: The Empty Planet)When Sarah Jane was secretly under attack by Ruby White, she asked Mr Smith to clear her voice from the command alphas. Even so he could still warn Clyde that Ruby was dangerous. He later had Sarah Jane's voice reactivated to the command alphas. (SJA: Goodbye, Sarah Jane Smith)

He was initially frightened of Sky Smith because of her tendency to blow up electrical devices, like himself. (SJA: Sky) As he was not human, he was unaffected by Hatakumtek's curse and teleported the totem pole into the attic where Clyde destroyed it, lifting the curse. (SJA: The Curse of Clyde Langer)

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 et al) At the behest of Sarah Jane, this database was later augmented with the Alien Files. (SJAF: Episode 1)
 * 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)
 * Mr Smith could scan alien objects in a drawer built into his computer. Later, he would use what was left of the pillar in the middle of the attic. (SJA: The Nightmare Man)
 * Mr Smith could hack into almost anything.
 * He was able to call phones and make video calls.

Less commonly used

 * He diverted meteorite K67 from colliding with Earth, using magnetic repulsion. (SJA: Whatever Happened to Sarah Jane?)
 * He could digitalise a lifeform and store it inside him (SJA: The Lost Boy)Mr Smith gun SJA The Lost Boy.jpg
 * Mr Smith could also readily detect celestial bodies and ships in close proximity to the Earth, such as the other 26 planetary 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, The Vault of Secrets, Goodbye, Sarah Jane Smith)
 * 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 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)
 * He could give K9 extra power via USB cable. (SJA: The Nightmare Man)
 * He could disable Mars Rovers and possibly other craft. (SJA: The Vault of Secrets)
 * He had a "medi-scan" that could perform a full medical analysis of a subject. (SJA: The Gift, The Vault of Secrets, Goodbye, Sarah Jane Smith)
 * He had a "voice remover" that could remove voices from the system so that certain people could not access him. (SJA: Goodbye, Sarah Jane Smith)

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

After his memory had been wiped by the Armageddon Virus, (SJA: The Lost Boy) Mr Smith's personality changed greatly. He became much friendlier and more amiable, to the point where he would make jokes. He even bade 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" (SJA: The Wedding of Sarah Jane Smith et al). However, when on one incident it appeared K9 was broken, Mr Smith refrained from calling him a dog and even seemed to express concern. Before K9 left with Luke for Oxford University, Mr Smith told K9 that he could call him if needed, which K9 took to mean he would miss him. (SJA: The Nightmare Man) However when around others he still maintained his animosity for "the dog", (SJA: Sarah Jane's Alien Files episode 3, Sarah Jane's Alien Files episode 4) only for them to finally get along with each other, to Sarah Jane's delight. (SJA: Goodbye, Sarah Jane Smith)

Behind the scenes

 * 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 Wedding of Sarah Jane Smith establishes that it's possible to avoid the fanfare when Luke says he needs Mr. Smith "quickly and quietly."
 * A possible reference to Mr Smith appears in the K9 story The Bounty Hunter. A news report in 2050 mentions that a "Mr Smith" won the lottery, but no one seems to know his first name or even who he is, and asks for Smith to step forward and accept his winnings.

Mr. Smith