When a prominent politician is somewhat mysteriously, but apparently accidentally, drowned while scuba diving, the press focus on the young, charismatic Senator Nick Rast (David Hemmings) who seems strongly favored to succeed the dead man and, by doing so, raise himself up another rung on the ladder of political power.

Rast, whose marriage to Sandra (Carmen Duncan) was a political convenience, nevertheless shares his spouse’s concern over their young son Alex (Mark Spain) who is stricken with leukemia. While doctors hold out no hope for the child, Alex is visited by a strange, flamboyantly mystical faith healer, Gregory Wolfe (Robert Powell) whose administrations in the presence of his parents effect a miraculous recovery in the boy.

Wolfe’s appearance at Rast’s closely guarded mansion is made all the more astonishing when the Senator realizes he has managed to evade all the elaborate security systems in gaining entry.

Gradually Wolfe becomes part of the family circle and as Alex continues to gain strength and abnormal powers from his instruction and philosophy, Sandra falls more and more under his spell. However Rast’s political backers, personified by the ruthless Doc Wheelan (Broderick Crawford) view Wolfe in a very different light. They begin to plot against him, worried by Sandra’s increasing dotage and dependence and her growing refusal to present a happily married front to the media. Even Nick recognizes that Sandra’s initial gratitude to Wolfe has turned to love.

Wheelan, with the aid of a carefully prepared dossier, manages to convince Nick that Wolfe is a fraud, despite the Senator’s own, first-hand witnessing of Wolfe’s remarkable powers. Wolfe is imprisoned but when Sandra learns what is happening she leaves her husband, taking Alex with her.

On the night he is due to receive a call confirming his new political appointment, Nick is confronted at the mansion by Wolfe, now dressed as a Harlequin, the magical character of Italian classical drama.

Wheelan and his men, frightened by Wolfe’s incredible jail-break, but worried by the danger to Nick and the threatened exposure of their own scheming, hurry to the house intent on destroying Wolfe.

After terrifying Nick with a demonstration of his powers, Wolfe convinces him of the truth. Nick discovers he has been used as a pawn by Wheelan and the drowning was no accident but part of the same plan to have him promoted as cover for Wheelan’s own power lust. But Nick is in too deep. He lures Wolfe into the kitchen where Wheelan’s marksmen can complete their murderous task.

But Harlequin still has another appearance to make …