A popular song in the 1960s quipped, “England swings like a pendulum do,” but during that flashy decade of youthful rebellion, there was also a much more sinister movement sweeping the land toward neo-Nazi white-supremacy fascism. PBS’s Masterpiece revisits this uneasy time (with unfortunate echoes of the present) in the tense four-part limited series Ridley Road, featuring the exploits of a most unusual Mata Hari–style spy.

We meet Vivien Epstein (an alluringly vulnerable Agnes O’Casey) as a restless young Jewish woman in Manchester, fleeing a dull arranged marriage proposal to follow her dashing boyfriend, Jack (Tom Varey), to London, though he warned her not to. Finding work as a hairdresser in fashionable Soho, this romantic is in for a rude awakening when she discovers that Jack has gone underground as a member of the anti-fascist 62 Group, based in the East End’s Ridley Road and led by her uncle Soly (Ray Donovan’s wonderfully combative Eddie Marsan).

Jack’s assignment was to infiltrate the notoriously anti-Semitic National Socialist Movement, and when he mysteriously disappears, Vivien impulsively follows suit, transforming herself into an unlikely undercover agent. Sounds like The Girl From U.N.C.L.E. but it’s way more serious.

Having been told by her hairdresser boss, “You could do with a more groovy look,” Vivien suddenly has platinum hair and a new persona—call her Jane Carpenter. Before long, “Jane” has captured the fancy of real-life fascist leader Colin Jordan (the chilling Rory Kinnear).

“You have the most intelligent eyes I’ve ever seen,” Jordan appraises as Jane/Vivien begins her campaign of seduction, appealing to his prejudices and ego and befriending his adorable young son. Hers is a dangerous double life, trying not to blow her cover while seeking news about Jack and acquiring evidence of the NSM’s covert military plans.

Ridley Road, based on Jo Bloom’s novel and inspired by actual events, captures the “mod” ’60s period effectively with the use of vintage footage between scripted scenes. The menace builds slowly but strains credibility toward the end when Vivien goes full Jane Bond, crawling through attics and scaling rooftops to escape Jordan’s hatemongering world, burdened by a bulging suitcase full of damning secrets. As melodrama it’s enjoyable, though ultimately a too-predictable escapade in the ongoing fight between good and evil.

Ridley Road, Limited Series Premiere, Sunday, May 1, 9/8c, PBS (check local listings at pbs.org)