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. 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 for him, Mr. Smith began to help Sarah Jane, and gave her knowledge on many things.

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
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 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 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)

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 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)

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 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, and he found it harmless. However, the morning after, the weed mutated and turned harmful, attacking Luke Smith and Sarah Jane. Mr. Smith used his cooling airvents to suck the weeds away. (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)

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)
 * 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)

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, The Vault of Secrets)
 * 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))
 * Giving K9 extra power via USB cable. (SJA: The Nightmare Man)

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 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". 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)

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. The name may also 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. 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