On an ordinary evening, a mother and father sing nursery rhymes to their daughter Helen before bed. Their teenage son Robert becomes alarmed when all the clocks in the house stop working. He hurries upstairs to find that his parents have vanished. Left alone with Helen and no neighbors nearby, Rob calls the police for help. But a pair of strange, time-traveling agents named Sapphire and Steel arrive instead and inform Rob that they are his only hope in getting his parents back.

Sapphire and Steel determine that Helen's bedroom is the location of a time break caused by the recitation of nursery rhymes. After narrowly avoiding a disaster, Steel boards up Helen's door to prevent the creatures inside from breaking loose. Rob begins to distrust the mysterious agents and sees a chance for escape when a policeman finally arrives in the morning. Sapphire is one step ahead of him, however, and traps the policeman in a time loop to prevent interference.

Steel's drastic measures to save Sapphire and trap the pool of light prove effective, if extremely dangerous. Sapphire places the pool of light in a freezer to keep it from escaping again and tends to Steel's recovery. She also enlists Rob and Helen's help in removing all the objects in the house that could become hiding places for other pools of light. Sapphire and Steel welcome the arrival of fellow agent Lead, a gentle giant with a huge personality to match his huge stature.

The time break strengthens after a nursery rhyme hidden in a book becomes stuck in Helen's head. Sapphire burns the nursery rhyme book and Steel and Lead barely succeed in stopping the time break from spreading. The kitchen is left a windblown mess and Helen and Rob help Sapphire with the clean-up effort while Lead and Steel keep watch upstairs. Everything takes a turn for the worse when two more pools of light escape from the bedroom and Rob suddenly goes missing.

Lured by a vision of his father, Rob walks into a trap in the basement. A pool of light has taken him back in time to the 1700s when the house was first being built. Sapphire and Steel fear the worst when they discover that all three pools of light are there in the trap with Rob. However, Steel figures out that the creature that controls the pools of light has made a mistake and decides to gain the upper hand by using Helen as bait. Sapphire objects but follows through with the plan because it is their last chance to save Rob, bring back his parents, and return ...