This series was based around the struggle of two timeless, feuding sorcerers - the noble Rothgo (Ron Moody) and the evil Belor (Pamela Salem).
Each character aimed to obtain possession of the Nidus, a magical object of limitless power. The Nidus actually belonged to Rothgo and was the source of his power, but it was stolen by Belor who used her own magic to send it through time and space so that Rothgo would not be able to find it. Without the power of the Nidus, Rothgo would ultimately die.
The first two seasons followed a group of modern-day children (Phil, Helen and Terry) who find Rothgo, almost lifeless, in a labyrinthine cave. Rothgo sends the children through different periods of time to search for the Nidus, which is disguised as a different object in each time period and can only be seen in reflection.
The children arrive at various points of history (the French Revolution, Ancient Greece, English Civil War, etc.), however their attempts are constantly thwarted by Belor who also appears in each time period, and who uses her magic to hurl the Nidus further through time to prevent Rothgo and the children from obtaining it (using her magical incantation "I deny you the Nidus!" as a bolt of lightning emanates from her fingertips).
Belor cannot actually touch the Nidus whilst Rothgo still lives, and so she can only hope to deny him possession of the artefact until he eventually dies. At the end of the first series, the children locate the Nidus (now in the shape of a sword) and finally manage to outwit Belor and return the Nidus to Rothgo, leaving Belor defeated.