"All irregularities will be handled by the forces controlling each dimension." So begins one of the most enigmatic and chilling horror series ever broadcast by ITV. This is ita story.

"...Sapphire and Steel have been assigned.” #SundayThoughts

Britain's ITV network had a strong reputation for children's paranormal drama in the 1970s: Timeslip, Ace Of Wands and The Tomorrow People all featured people with extraordinary powers tackling mysterious foes.

Against this background writer Peter J. Hammond pitched the idea of a new children's show - initially called The Time Menders - to Thames Television. They declined, but rival ATV were impressed by the strength of the scripts and took the project on.

ATV renamed the show and suggested itshould have a later time slot that could handle more mature themes. They also wanted recognisable actors as the lead roles, in the hope of selling the show in America through Lew Grade's ITC Entertainment company.

David McCallum and Joanna Lumley were recruited to play the lead roles: inter-dimensional operatives called Sapphire and Steel, who took human form in their mission to stop time itself wreaking havoc on the present.

The premise of Sapphire and Steel is ambiguous: 'elements’ are assigned by a higher power to prevent time from exploiting anomalies in the fabric of the universe. Sometimes beings from the past or future are the threat; sometimes it's time itself.

Sapphire is the more empathetic agent: she can rewind time in small ways to see what has happened, and can tell the age and history of objects or people by touch. Often her eyes change colour when this happens.

Steel is a more cold, calculating character. He has enormous strength and can freeze himself to near absolute zero, allowing him to destroy paranormal entities. He has little sympathy for humans.

Other 'elements' occasionally help Sapphire and Steel: Lead has immense strength, whilst Silver is a mischievous Technician who can manipulate technology and create holograms. He has rather a thing for Sapphire too.

Sapphire and Steel isn't a straightforward time-travel drama. Time itself - and various other dark forces - are the ‘villain’ constantly trying to break into the present to disrupt it. The 'elements' don't care much for humans caught up in this, only with keeping time in check.

The thing that's gripping about Sapphire and Steel is its claustrophobic sense of terror: the sets are sparse and confined; the plot proceeds slowly like a good horror novel, building up the tension.

Stories (called 'Assignments') weren't given titles, so the audience had no clue what was going to happen. Assignments 1 and 2 were broadcast in 1979, though an ITV strike disrupted the planned schedule.

In Assignment 1 nursery rhymes told in an old house allow time to force itself into the present; plague victims and Roundhead soldiers terrorise the children whose parents have vanished. Sapphire becomes trapped inside a painting, while something awful lurks within the cellar.

In Assignment 2 a ghost hunter in an abandoned railway station meets 'Darkness', a time being who feeds on the resentment of the dead who haunt the station: people who unjustly died, such as a WWI soldier shot minutes after the armistice.

Ratings for the first two Assignments were high, and so three more Assignments were recorded and aired in 1981. A final, shorter 'Assignment 6' was also filmed as a contingency.

Sadly Sapphire and Steel had very little budget: scripts made use of whatever sets and props happened to be available at the time. David McCallum and Joanna Lumley couldn't commit to the filming schedule, and so the show ended in 1982.

Assignent 6 was a great send-off: a motorway service station filled with strangers from different times. One stands and announces "this is the trap. This is nowhere, and it’s forever." They vanish. Sapphire and Steel are left in an empty room floating outside of time itself...

The show was genuinely frightening, as well as maddeningly cryptic: what was happening was never fully explained. Instead the viewer was caught up in the mood of horror and dread, never sure what might happen next. Samuel Beckett and JB Priestley were both clear influences.

ITV did try to market Sapphire and Steel to children(!) with a comic strip in Look-In magazine and a Christmas annual. And in 2004 Big Finish Productions acquired the rights to make a number of audio adventures with Susannah Harker and David Warner in the lead roles.

Will there ever be a Sapphire and Steel TV reboot? I hope not; it's from an age of slower stories and ambiguity, when a good scare was more important than a witty putdown. Sapphire and Steel: Twitter salutes you - wherever you are!

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