The Mystery of the Valley of Giants!

A gang of cattle rustlers descend on the Lathrop ranch to steal the entire stock. They are held off by Jim Lathrop, his daughter Marie and a posse. They are assisted by the sudden arrival of the Black Rider who guns down the rustlers and leaves after giving a kiss to Marie.

Meanwhile, in Chicago, a scientist named Professor Chalis is being reprimanded by his fellow scientists for his grizzly experiments in growing humans to the size of giants. They question his unethical science and that it could mean the destruction of the human race. Chalis refuses to listen to reason and tells them that he will continue his work. Deciding that his colleagues will try to stop him, Chalis decides to relocate to the American west where he can continue his experiments uninterrupted. He returns to his lab to find his giant, Zugg, has killed the police officers who had come to clear out his equipment. Lamenting how his growth experiment causes a loss of intelligence in those exposed to it, Chalis has Zugg assist him in fleeing town, intend on continuing his experiments and correcting the flaw.

Six months later, Dr. Matthew Masters is met by one of the Lathrop ranch hands named Jim who tells him about Professor Chalis who moved into the area nearth the Lathrop ranch recently, however Masters has not had the opportunity to meet him yet. Jim also tells of how he hears strange noises coming from Chalis' ranch. Masters is dismayed to see that Chalis has already met and is riding into town with Marie Lathrop, who seems to find the scientist fascinating. When Matthew confronts her about it, Marie once more chastises him for not being interesting like Chalis or brave like the Black Rider. Suddenly, Bobby rides into town and tells Marie that some of their bulls have been stolen overnight, so Marie and Jim ride off back to the ranch.

At Bobby's suggestion, Matthew slips away to investigate himself. Looking around the area where the bulls went missing, Masters finds a massive foot print that looks human and decides that it's time for the Black Rider to start investigating. Changing into his costume, the Black Rider follows the footprints out to Professor Chalis' lab. However, when he tries to bust in he is attacked by a giant wolf and guns it down then flees before he can be discovered. The Black Rider changes back into his civilian guise and returns to town and tells Bobby what happened then goes to the local library to read up on Professor Chalis and learns about his growth experiments.

Meanwhile, at Professor Chalis' lab, the Professor has Zugg train his growth ray on the captured bulls. However, before the experiment can be completed, Chalis has everything hidden when Marie comes by for a visit. The Professor then shows off his work, but when Zugg comes out of hiding, he frightens Marie who accidentally bumps the activation switch of the growth ray. The ray beam strikes the Professor, who then shuts it off before passing out. Marie rushes out and gets Doc Masters and brings him to the ranch. Masters is shocked to see the Zugg and realizes what Chalis has been up to. Doc then treats Chalis and then slips away to look around as the Black Rider. Searching, the Rider accidentally activates a trap door and is almost dropped into a pit filled with giant rattlers. Zugg tries to attack him, but Black Rider breaks free and flees, changing back into Doc Masters. He finds Chalis seemingly all right and so he and Marie leave.

However exposure to the ray has driven Chalis mad and he creates an antidote to reverse the growth effects so that he can experience them. A few days later, Masters returns to his office and is shocked to find Jim there, and that he has been experimented on by Professor Chalis, and has grown to a giant size but lost intelligence as a result. Meanwhile, Marie and Bobby are out on a ride when they run into Professor Chalis. When Marie refuses to go back to Chalis' manor, he kidnaps her and rides away. Bobby then rushes to Doc Masters' office.

As the Black Rider, Masters rides out to the Professor's manor where he battles Zugg. When he realizes Marie is tied up, he weakens his focus and is defeated by Zugg and taken prisoner as well. When the Black Rider comes around he finds himself and Marie in the path of the Professor's growth ray. However, before the Professor can activate it, his giants turn on him destroying the machine and killing Chalis. As the fire spreads, Satan bursts through the door and chews through the ropes that keep the Black Rider and Marie prisoner. When they try to escape, Zugg gets in the way but Satan kills him.

With everyone free, the Black Rider gives the antidote to the other giants and then bids everyone farewell and rides off into the night.

Recurring Characters

• Black Rider

• Marie Lathrop

• Jim Lathrop

• Bobby Lathrop

• Satan

The Spotted Invader!

Nimo defends his domain from a Jaguar.

Recurring Characters

• Nimo