Idaho 1889: The region wants to be given statehood, which would mean it would fall under US-jurisdiction, but there are still forces who try to use the region's somewhat vague legal state to their advantage - and statehood would mean the end of their operations and criminal prosecution.
Newspaper man Randolph Meredith (Jay Kirby) is one of the men literally cruzading for statehood, but not only by publishing articles in his newspaper but also by fighting outlaws in a Zorro-like disguise, as the Black Whip. Thing is, Meredith is shot dead before even chapter one of this serial is over, so it's up to his sister Barbara (Linda Stirling) to put on his costume and fight injustice in his stead. Before long she finds an ally in gouvernment agent Vic Gordon (George J.Lewis) - he is described as an undercover agent, even though he never does any undercover work -, however, she doesn't disclose her secret identity even to him, and ultimately only Tenpoint (Lucien Littlefield), the printer of her newspaper, knows that she really is the Black Whip.
The biggest opponent to the Idaho-statehood is Hammond (Francis McDonald), your typical honourable citizen on the outside but really the leader of an outlaw gang who want to loot Idaho for all it's got. From here on it's shoot-outs and chases aplenty, with Vic and Barbara - in costume and out - escaping many a close call. Eventually Hammond even figures that Barbara has to be the Black Whip, but by that point, Vic has independently found out her secret, donned her costume and freed her in disguise, thus diverting suspicion from her. Utimately, Vic and Barbara find out that Hammond is really the leader of the outlaws and drive him out of town - but he's not one to be easily defeated, and on election day for Idaho's statehood he plans to steal the ballotts to get his hands on Idaho after all. However his men are in the end outnumbered by the Black Whip, Vic and a bunch of gouvernment agents, and he alone escapes. Upon his escape though, he discovers the hide-out of the Black Whip, sees Barbara unmasking and has her at gunpoint - when her horse goes wild and stomps him to death.
Some of Republic's serials, especially the early ones (e.g. Darkest Africa, Undersea Kingdom, The Adventures of Captain Marvel), were great, and be it for their cinematic merits or (more often) their camp value. Zorro's Black Whip on the other hand is only just mediocre: Its storyline is strangely uninvolving and lacks dramatic build-up, the episodes themselves are pretty much interchangeable and don't seem to have any effect on the story as a whole, the action is just average, the direction is functional and the bheroic duo of Linda Darnell and (especially) George J.Lewis is nothing short of totally bland. This does of course not mean that the serial is all bad, it's very slickly made (a bit too slickly for my taste), is decently budgeted, and some of the cliffhangers are even exciting. Just don't expect too much.
By the way, the credits boldly state that this serial is somehow based on Johnston McCulley's Zorro character, but besides the fact that it also features a masked crimefighter who rides a horse, there are no similarities, neither in location (California vs Idaho) nor in weapon of choice (rapier vs whip), and not even in gender (male vs female, obviously).