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The series involves them, in their capacity as members of the Intrusion Countermeasures Group (ICMG), solving mysteries involving paranormal, supernatural, anachronistic or alien phenomena in Britain in 1964, although the production team stated in the third series' Behind the Scenes that they had limited the number of alien incursions featured. Simon Williams, Pamela Salem and Karen Gledhill reprised their roles as Gilmore, Rachel and Allison respectively.
They were joined in the regular cast by Hugh Ross as Sir Toby Kinsella, the ICMG's liaison with the British government. The series was planned from its conception to be four series long, and the developing arc was concluded with the final box set.