Mr Smith

Mr Smith was a super-computer (of sorts) owned by Sarah Jane Smith. The computer was located in the attic of Sarah Jane's house at 13 Bannerman Road.

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. Decades later, Sarah Jane Smith retrieved the living crystal, Sarah Jane took in her home on 13 Bannerman Road and found it was able to communicate with her via her laptop computer. It built itself a body out of machines. This way, the crystal informed her as to 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 caused him progressive amnesia. Using this to her advantage, Sarah Jane informed him of his mission to serve humanity. (SJA: The Lost Boy)

After Reprogramming
Mr Smith relayed the Dalek message to Sarah-Jane and Luke. He boosted the signal from Martha Jones's phone in conjunction with the Rift Manipulator in order to contact the Doctor who was outside of the Medusa Cascade as a result of the Earth being stolen. The computer appears to take damage from this effort, and its fate is unknown. (DW: The Stolen Earth)

Abilities

 * Mr Smith apparantly plays a fanfare of his own accord when activated, as shown when Sarah Jane Smith requested that he stop playing it during his activation following the transportation of Earth to the Medusa Cascade.
 * Mr Smith can also readilly 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.
 * Mr Smith can relay messages from alien sources.
 * Mr Smith possessed knowledge of and kept a database on many alien races, and secretly monitored the activity and communications of alien ships.

Aliens Featured

 * Krillitane
 * Bane
 * Butterfly Person
 * Uvodni
 * Raxacoricofallapatorian
 * The Beast
 * Abaddon
 * Macra

Behind the scenes

 * Mr Smith is presumably named for John Smith, the often-used alias of The Doctor (and one he used in his tenth incarnation when he met Sarah Jane for the first time in years). (DW: School Reunion).