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Dr. Sweetman was a UNIT medical officer with whom Liz Shaw was assigned to work on medical matters alongside her initial role as UNIT's scientific advisor. According to Sergeant Benton, Dr. Sweetman was one of the few people within UNIT who officially knew of the existence of the Glasshouse. (PROSE: The Scales of Injustice)
According to one account, Dr. Sweetman had maintained the medical officer position by the time of the events leading up to the Third Doctor's regeneration. Upon discovering that the Doctor had fallen into a trance-like state after handling a Metebelis crystal, Brigadier Lethbridge-Stewart called Dr. Sweetman over the phone, asking them to come to the laboratory immediately. However, when the Doctor was revived by the smell of Sergeant Benton's coffee, the Brigadier cancelled the request for Sweetman's assistance.
Later, when Lupton infiltrated UNIT grounds and asked Corporal Hodges about the Doctor's whereabouts, Hodges initially believed that Sweetman was the doctor to whom Lupton was referring. (PROSE: Doctor Who and the Planet of the Spiders)
However, according to another account, it was instead Harry Sullivan who held the role of UNIT medical officer at this time, with Sullivan being the one whom the Brigadier contacted when the Doctor fell into a trance, and the medical officer mentioned during Lupton and Hodges's exchange going unnamed. (TV: Planet of the Spiders)