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K-9 first appeared in the TV episode 'The Invisable Enemy'. K-9 mark 1 was the robotic dog of Professor Marius. After defeating the alien creature known as The Nucleus the Professor gave K-9 to the Doctor as a gift. After that K-9 mark 1 travelled with the Doctor and Leela until the TV episode 'The Invasion of Time' when K-9 decided that he wanted to stay with Leela on the Doctor's home planet Gallifrey. When the Doctor left them he pulled out a box from storage. Within it was none-other than K-9 mark 2.
K-9 mark 2 travelled with the Doctor and Romana and helped them gather the parts of The Key to Time, when Romana regenerated she eventually left the Doctor and K-9 went with her through warriors gate leaving the Doctor with Adric. After that the Doctor no longer travelled with K-9, but gave mark 3 as a gift to Sarah Jane. Years later K-9 Mark 3 was destroyed saving the Doctor and Sarah Jane, but the Doctor gave her a new K-9, Mark 4.
K-9 mark 3 has featured in the spin-off adventure 'K-9 and Company', while K-9 Mark 4 features in the TV series The Sarah Jane Adventures.
K-9 has also got his own spin-off TV series. It features the adventures of K-9 mark 1 who has come through space and time following a reptillian alien to Earth. To save the humans he is forced to self destruct but in his remains a boy who he has saved finds his regeneration backup, and thus we now have the new and improved.....