The programme centred on a pair of interdimensional operatives, the titular Sapphire and Steel. Very little was revealed about their purposes or backgrounds in the course of the series but they appeared to be engaged in guarding the order, if not the integrity of Time. In the series, it was explained that Time is like a corridor that surrounds everything, but there are weak spots where Time — implied to be a potentially malignant force — could break into the present and take things. There were also creatures from the beginnings and ends of time that roamed the corridor looking for the same weak spots to break through.

 

These breaks were most often triggered by the presence of an anachronism, for example a nursery rhyme, a doctored photograph that mixed period and contemporary elements, or a house decorated to replicate a 1930s setting. Investigators would assess the situation and then, if intervention was warranted, Operators were assigned to deal with the problem by a mysterious unseen authority, to be assisted by Specialists if necessary.

 

The stories were generally quite cryptic, raising more questions than answers, and had an eerie air to them, being as much ghost stories as they were science fiction. The programme had been allocated a minuscule production budget, which led to the use of simple (but very effective) staging and minimal special effects, ultimately contributing to the uneasy atmosphere of the show. The ambiguous nature of the programme extended to its main characters. While Sapphire was portrayed as more affable and "human" than the no-nonsense, grim Steel, it was clear that their prime concern was to deal with the break in Time, sometimes over the safety of the humans caught in the incidents they investigated.