M is for... Mr Smith

Sarah Jane Smith's super-computer, which is housed behind a false wall in her attic study. When summoned with "Mr. Smith - I need you", the machine would emerge, accompanied by a musical fanfare which Sarah used to find annoying.

Initially, its origins were unknown - other than it had been found in the vicinity of the volcano Krakatoa. Sarah had received a crystal anonymously in the mail, and whilst examining it she found that it could interact with her home computer, greatly improving it. It then showed her how to construct its full self.

Naming it "Mr. Smith", Sarah found it to be of invaluable help. Not only could it assist with any terrestrial matter, it held a vast databank on alien phenomena.

Little did she know that it was concealing a dreadful secret. It made contact with a group of Slitheen who sought revenge against Sarah for the death of some of their kin. First it faked a story about Sarah's adopted son Luke - making out that he was really an abducted Earth boy. With Sarah distracted by these events, the Slitheen then planned to drag the Moon into a collision course with the Earth.

The crystal at the core of Mr. Smith was really a ruthless alien Xylox, which had been trapped under the ground since prehistoric times. The collision would free more of its kind still hidden there.

Sarah's friend Clyde discovered that the computer was acting against them, and it had him converted into data trapped within itself. He was able to communicate via another computer with Maria Jackson and her father.

Luke helped foil the plot - along with the Slitheen, when they realised that they would be destroyed too. 

Sarah then had Mr. Smith's memory wiped with K-9's help.

The computer would later have a love-hate relationship with the robot dog.

On two occasions, whilst under malign alien influence, Sarah had Mr. Smith shut down to prevent it from interfering with their schemes.

When the planet was transported across space to the Medusa Cascade by the Daleks, Mr. Smith was used to boost the world's phone signals to contact the Doctor.

Voiced by: Alexander Armstrong. Appearances: The Sarah Jane Adventures (Series 1 - 5), The Stolen Earth / Journey's End (2009).

• Armstrong would later feature in Doctor Who in person, playing bomber pilot Reg Arwell in The Doctor, the Widow and the Wardrobe.
• For some time, viewers assumed that the fanfare which accompanied Mr. Smith's emergence was simply part of the incidental music - until Sarah mentioned that it was actually heard by those in the attic.