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Dekker was the head of MI5's technology division, and was also in charge of alien monitoring. Dekker had been constantly monitoring the airwaves for the 456 from the 1960s up until their arrival in 2009 and knew far more about the aliens than anyone else.
In 2009, Dekker discovered that once again the 456 had contacted the world via the airwave on an old computer from 1965. Almost immediately he contacted civil servant John Frobisher, telling him about the 456's return. Dekker told Frobisher he had been too busy working to have children.
Dekker oversaw the creation of the tank for the 456 in Thames House. He was with Frobisher during their first conversation with the 456 ambassador.
When Captain Jack Harkness angered the 456 and it released poisonous gases into Thames House, Dekker escaped to his office and put on a HAZMAT suit which allowed him to breath non-contaminated air, making him the only person in the building to survive as Captain Jack died, albeit temporarily.
Dekker greeted Colonel Oduya of UNIT, who took over the negotiations with the 456. Later when a small mutiny led by Agent Johnson took place, Captain Jack was released from prison and Dekker was knocked out and taken to Ashdown House, where he was shot in the leg by Johnson after he complained.
Dekker informed Harkness that the child at the centre of the wave to destroy the 456, which would have to be Harkness' own grandson, Steven Carter, would be killed in the process, laughing when he said "that child's gonna fry". Steven did die during the process and Dekker was saddened at the horrific death.