Assignment III: The Creatures' Revenge (5) (S03E05)

Sapphire and Steel manage to capture the changeling and discover the location of the time source concealed within the wall. Lacking their technician, they struggle to dismantle the wall and confront the creature lurking inside. Steel decides to take bold action, and their gamble succeeds when the couple reemerges in the capsule, transforming the changeling back into an infant. However, the entity hidden within the wall continues to flourish, revealing a more significant danger than they had anticipated.