A wealthy business man holds a party in a 1930's period reconstruction for his guests, but all is not what it seems. Sapphire and Steel invite themselves to investigate.

A game of hide and seek is played before dinner but one of the guests will be murdered, there is something evil hiding in one of the guests but which one?

The party guests begin to realize that they are trapped, but their memories are beginning to fail and confusion is taking over. Then there is another death. But is there really?

Two of the party guests are dead. Who will be next? This is no traditional murder mystery, and Sapphire and Steel cannot agree on who would have a motive to commit this crime.

Sapphire and Steel realize that things must play out exactly as before in 1930 or the human race is doomed, but something is trying to interfere with time. Dr McDee has to die.

Sapphire and Steel allow things to unfold to find the point in time when things will be changed. Only then can they ensure that time is put back on track and reset the correct future.

Sapphire and Steel receive a puzzling assignment at an apartment complex where time is poised to attack. However, their inspection of every room in the complex reveals nothing suspicious. Even the abandoned top floor reveals no indications of trouble. It is only after investigating the roof that Sapphire and Steel discover an invisible time-capsule from 1500 years in the future. A couple and their new baby are living inside the capsule as an experiment, but they are in grave danger because of a horrible figure hidden inside the capsule with them.

Rothwyn and Eldred, the couple living inside the space capsule, begin to realize they might be in serious danger when they cannot make contact with fellow study groups or with their superiors. The evil creature inside the capsule begins an attack by using time to transform the couple's infant into a grown man. As Sapphire and Steel deepen their investigation, Sapphire discovers that the mechanism which transported the capsule is no ordinary machine.

After Sapphire disappears while trying to contact the creature inside the capsule, Steel must figure out where she has gone and how to proceed with the investigation. Luckily, technology specialist Silver arrives to lend a hand with getting inside the capsule. Rothwyn and Eldred discover their child has gone missing and are at a loss for what to do next. Meanwhile, their adult-sized infant wanders the capsule under the control of the creature hidden inside the wall.

Once Silver concocts a way inside the capsule, he transports Sapphire and Steel into the experimental pod from the future. The couple are no where to be found but Sapphire, Steel, and Silver come face-to-face with the adult-sized infant. Sapphire deciphers that the overgrown baby is a changeling controlled by time. The changeling senses that the three time agents are a threat and the force controlling the changeling commands that he destroy them.

Sapphire and Steel subdue the changeling and find where the time source is hiding in the wall. Without their technician, however, they have no way to deconstruct the wall and defeat the creature hidden inside. Steel's decision to resort to drastic measures pays off when the couple reappear in the capsule and the changeling becomes an infant again. But the creature inside the wall still thrives and presents a larger threat than anyone could have imagined.

After Silver's return, the three time agents question the couple from the future about their origins and their knowledge about the machine that transported them through time. The couple reveals some horrifying information about the future that directly relates to the creature inside the wall. Now with an understanding of the creature's motivation, Sapphire and Steel must prevent the creature's escape and manipulate time in order to save all of humanity.

Sapphire and Steel investigate strange happenings at a pawn shop. Children from the past are appearing, but how are these children connected to old photographs? What does the single tenant in the upstairs room know?

Who is the new Landlord? He confronts Sapphire and Steel but his face is not what it seems. He warns them to leave or suffer the same fate as the missing shop owner.

The missing shop owner and missing former roommate of the tenant are found in an unexpected place. Sapphire tries to contact the missing roommate. The faceless man intervenes.

Trapped Sapphire and Steel must use their wits and the help of the tenant to escape their prison and trap the faceless man and keep him from causing more harm.

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.

Sapphire and Steel investigate the upstairs hallway after Rob sees a pair of ghostly soldiers on the landing. Unbeknownst to them, a living pool of light has escaped from Helen's room and now lies in wait in a portrait at the top of the stairs. Sapphire walks right into the trap and disappears from the house. Only faintly able to communicate with Steel and Rob, Sapphire tells them she is stuck in a freezing cottage with the sensation that something sinister is there with her. Steel and Rob discover that Sapphire is trapped in the portrait and must work quickly to avoid losing her.

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 ...

For their new assignment, Sapphire and Steel arrive at an abandoned railway station haunted by supernatural figures. Paranormal investigator George Tully has been observing the station for several weeks because he has an interest in speaking with the ghost of a young man often heard whistling in the hallways. Sapphire and Steel indulge Mr. Tully by allowing him to help their investigation but Steel warns him that the apparition is something far more dangerous than a ghost.

Sapphire and Steel discover the ghost is a young World War I soldier who died in battle. They begin to suspect the railway station might be a recruitment ground for the dead. The appearance of a second ghost and the discovery of a recording with the last words of three dying submariners all but confirm their suspicions. The investigation becomes even more difficult and dangerous when the ghost of a fighter pilot forces Steel to experience a deadly plane crash.

After Sapphire narrowly saves Steel from meeting the same fate as the doomed fighter pilot, Steel becomes determined to figure out why the ghosts are being recruited at the station. Since the ghosts have such strong emotions, Steel tries to anger the soldier into appearing by singing an old military marching song with Tully. Steel's tactic works a little too well and the soldier takes out his anger on Sapphire and Tully.

Steel continues to taunt the ghost of the solider but pauses when he finds Sapphire and Tully unconscious. A hallway in the railway station has become a re-creation of a submarine where three young men died from lack of oxygen. Sapphire and Tully nearly suffocate but Steel manages to carry them to safety before the three of them meet the same fate as the young submariners. Afterward, Sapphire and Steel agree to give Tully's methods a chance by holding a séance to find more information about the solider when he was alive.

With Sapphire as a medium, Tully and Steel are able to communicate with the ghost of a woman who was once the soldier's schoolteacher. As they prod the spirit with questions, she reveals that the force helping and controlling the ghosts is a terrible, living darkness. She also gives them the name of the soldier and the details of his untimely death. Satisfied with the information, Tully and Steel finish up the séance but an enveloping darkness creeps through the room and threatens to devour them all.

With Sapphire unconscious, Steel is unnerved by the appearance of her ghost on the railway platform with the other spirits. After Tully agrees to watch over Sapphire's body, Steel has a chat with her ghost to determine if she is real or a trick perpetrated by the darkness. The soldier is not through with trying to get rid of Steel, however, and snares him in barbwire. Meanwhile, the darkness makes a deal with Tully: he can escape the station unharmed if he leaves Sapphire and Steel while they are unconscious and vulnerable.

Sapphire and Steel awake to find they have been shunted forward in time twelve days. In that time, the darkness has gone and so have all the ghosts with it. A frightened Tully tries to convince Sapphire and Steel that he needs to leave the station for personal reasons, but they can guess he wants to leave because he made a deal with the darkness. While Tully's bargain failed, Sapphire reluctantly agrees to make contact with the darkness so that Steel can make his own deal with the enemy.

Sapphire succeeds in making contact with the darkness at a great personal risk. Steel negotiates with it by promising a powerful source of resentment in exchange for a return to present day. The solider finally reveals what it is the darkness has promised all the ghosts but Steel gives them a better offer. The darkness follows through with its part of the bargain and Sapphire and Steel must uphold their part as well, even if their actions damage time itself.

Reuniting the series stars after nearly thirty years, this documentary features Joanna Lumley and David McCallum reminiscing about the making of this landmark series. Also featured are Writer/Creator P. J. Hammond and Producer/Director Shaun O'Riordan