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Doctor Carter was a pathologist who was given the hand of Eldrad to examine after Sarah Jane Smith was brought to the hospital where he worked. Carter and the Fourth Doctor examined readings from an electron microscope that showed the hand was absorbing radiations.
Sarah came into his lab, used Eldrad's ring to render Carter unconscious and took the hand. When he woke, he drove the Doctor to Nunton Experimental Complex in pursuit of Sarah.
The use of the ring on Carter brought him under Eldrad's influence — in Sarah's words he had "seen the light of Kastria". In an attempt to prevent the Doctor from reaching Sarah, Carter attacked him, but his own momentum saw him fall several storeys down inside the complex to his death.