K9

K-9 mark I was a mobile computer in the shape of a dog, designed by Professor Marius, to replace his real dog left behind on Earth.  K-9 was a databank of considerable capacity.  He was also equipped with a blaster for offensive and defensive actions.   His blaster could kill, stun, or paralyze living beings, and melt or disintegrate metal.  K-9 was shown to be capable of regeneration.  He could interface with the TARDIS, the Matrix, and other machines.

K-9 mark I helped the Fourth Doctor and Leela against the Virus of the Purpose, which briefly infected him.  When Profesor Marius found out that he could not take K-9 back to Earth with him, he asked the Doctor to take him.  K-9 then accompanied the Doctor and Leela on their travels, and helped them fight the Sunmakers and the Oracle.   After helping the Doctor free Gallifrey from the invading Vardans and Sontarans, K-9 mark I chose to remain with Leela on the planet of the Time Lords.

The Fourth Doctor built a K-9 mark II, which had greater mobility and would respond to a silent dog whistle.  K-9 mark II accompanied the Doctor and Romana on the Quest for the Key to Time.  He fought the Graff Vynda-K, the Captain of Zanak's Polyphase Avatron, Vivien Fay's Orgi, and interfaced with the Mentalis machine, but was unable to take part in the battle on Delta Magna's moon because of it's swampy terrain.  Later, speech circuit defects (or space laryngitis) prevented K-9 from helping the Fourth Doctor and the newly regenerated Romana against the Daleks and Scaroth.  But he soon joined them against Lady Adrasta, the Mandrels, the Nimon, Pangol and the Foamasi, Meglos, the Marshmen (who severely damaged it) and the Great Vampire.  K-9 mark II was gravely disabled by the Time Winds during the Doctor's battle with Rorvik and elected to stay in E-space with Romana and the Tharils.

The Doctor sent an already assembled K-9 mark III as a present to Sarah Jane Smith.   He helped her defeat a coven of witches who worshipped the pagan goddess Hecate in Moreton Harwood.  K-9 mark III tried to warn Sarah Jane of Borusa's Time Scoop, but was ignored.