Travelling over the Bavarian Alps in the Fantasticar to investigate reports of mysterious beings in the mountains, the Fantastic Four come under attack from Gorgon, an Inhuman. Their ship is forced down and they come into contact with the rest of the Inhumans, led by Medusa. She tells them that their hidden valley is known as the Great Refuge, and they have used their powers to keep it hidden from humanity for many years. She informs the Four that the Inhumans intend to reveal themselves to the human world and then rule it. The Four are imprisoned and Medusa uses a hypnotic ray on Ben, getting him to serve her. The rest of the team escape their cell, and Ben is ordered to capture Herbie. After Ben tracks Herbie down, the robot gets Ben to hit his head on the floor, restoring his memory. Reed and Sue retrieve the Fantasticar, and after avoiding attacks from a missile, Gorgon and Karnak, meet up with Herbie and Ben. However Crystal, another Inhuman, uses her power over the elements to ground the Fantasticar. Seemingly trapped by the Inhumans, the Fantastic Four have to find a way to stop the Inhumans' plans of conquest and free themselves.

Herbie is helping Ben exercise using Reed's testing room, when a mysterious lady enters their headquarters and is captured by the automatic defense systems. She introduces herself as Queen Sebel of Manopal and wishes to enlist the help of the Fantastic Four. She uses a hypnotic influence on Ben to get him to agree to the trip to Manopal along with the rest of the team. There she tells them of how the Great White Stone, a large and valuable diamond was stolen by bandits. While acknowledging a few problems with Sebel's story, such as why no one else was sent to look for the diamond, the Four agree to track it down for her by looking in the surrounding jungle. They soon come into contact with the bandits themselves, who tell the team that Sebel must be overthrown. Sue retrieves the diamond, but before she can study it, Ben and Reed, now both under Sebel's influence, insist that it must be returned to her, and leave Sue and Herbie with the bandits. She asks them what they wanted with the diamond and they tell her that she used her powers along with the diamond to enslave the people of Manopal. Sebel gets the diamond back and forces Reed, Ben and Herbie to work as slaves, and transforms Sue into diamond. Forced to work in the mines, the remaining three of the team find Miel, Sebel's sister, who was imprisoned over a thousand years ago. Reed and the rest of the team, along with Miel, are left to figure out a way of freeing both Sue and the people of Manopal from Sebel's powers.

In Europe, a large power plant sinks into the ground, the latest such occurrence. The Fantastic Four are contacted for help, and Reed mentions that a mysterious individual known as the Mole Man is the one who is apparently responsible for the power plants' disappearances. After feeding him the necessary data, Herbie predicts the next plant to disappear will be Russia's largest power plant. The Four travel to Russia and witness the power plant sinking into the Earth's surface. After other plants are sunk into the ground, Herbie tracks the Mole Man's base to the legendary Monster Isle, and the team sets out to find him. Experiencing bad weather conditions, the Four are forced to land on the island, with Ben and Herbie ending up in the Mole Man's lair. He tells them of how he was once a respected scientist who was convinced of the existence of Monster Isle and was ridiculed because of it. He eventually found the island and discovered strange mindless creatures that regarded him as their god. Having dwelt in the darkness for so long also made him develop a radar sense which he demonstrates by avoiding one of Ben's attacks. With Ben and Herbie helpless, the Mole Man tells them that he intends to make the world defenseless while his subterranean armies take over the surface. Reed and Sue track Ben and Herbie down, and together they must stop the Mole Man's armies and return all of the captured power plants to the surface.

With nothing to do on a quiet night, Ben takes the Fantasticar out for a spin and observes a spaceship in the sky. The spaceship abducts Ben, and the rest of the team attempt to track the ship down. The spaceship travels to the dark side of the Moon, and the commander of the ship, Sandro of the Centaurians, introduces himself to Ben. He tells him that they have been at war for a long time with the Polarians, and to end the war, each side has chosen a champion to fight for them. The Centaurians have chosen Ben to fight for their planet's supremacy and force him into the contest by threatening the Earth. Meanwhile, the rest of the team have tracked Ben down and hitch a lift on a Moon rocket. Inside an air-tight dome on the Moon, the Polarians led by Pollus arrive with their warrior, Monstro, a giant humanoid lizard. Monstro and Ben then compete with each other in a series of events, while Reed, Sue and Herbie enter the dome. Sue turns invisible and swaps the keys of the respective alien spaceships around. Ben and Monstro then work together and shatter the dome, causing the air to leak out. Trying to escape, the Centaurians and Polarians are unable to open their ships. Reed tells them that unless they develop a trust for each other and hand the proper keys back over, they will all run out of air.

In Latveria, Doctor Doom plans to defeat the Fantastic Four before ruling the world himself. Doom travels to New York and ensnares the Baxter Building with a giant electrical wire. He reveals himself to the Fantastic Four, and Reed recognizes Doom. Reed tells the others of how he and Victor Von Doom were once classmates at college years ago and that Doom's forbidden experiments resulted in an explosion which scarred his face and resulted in his expulsion. Doom travelled the world in search of specialists to restore his face, but to no avail. He eventually ended up in Tibet and obtained many secrets of science and sorcery from a remote monastery, along with a suit of armor. Reed agrees to go to Latveria in order to face up to Doom's challenge, and the rest of the team go with him. In Doom's homeland, he tells the Four of how he needs more wealth in order to rule the world and gets Reed, Ben and Herbie to travel back in time to take the treasure of Blackbeard the pirate, while holding Sue hostage. Back in the past, the rest of the team dress up as pirates in order to be less conspicuous, and end up joining a pirate ship's crew. They search the ship and find the treasure in the captain's quarters. After defeating the crew, Ben is hailed as being the greatest of all pirates by the crew, who dub him Blackbeard. With the real identity of Blackbeard discovered, Reed has to figure out a way of keeping the treasure safe from Doom before they return to the future and to then rescue Sue.

In an isolated mansion deep in a far away forest, the super-villain known as the Wizard has invited Medusa, the Sandman and the Trapster. He tells them that they must work together as a team in order to defeat the Fantastic Four, and that they will be called the Frightful Four. Back at the Baxter Building, the Fantastic Four are alerted to a jewellery store robbery and leave to deal with it. Herbie detects the presence of the Wizard and his new team, and they carefully prepare to deal with this new menace. Reed and Sue deal with the Wizard and Medusa while Ben and Herbie are left to stop the Trapster and the Sandman. Not properly prepared to deal with the Frightful Four, the Fantastic Four are out of their depth and the Wizard's new team manages to escape from them. As the Fantastic Four have to deal with their very public defeat, the Frightful Four begin to argue among themselves before the Wizard tells them to follow his new plan. They lure the Fantastic Four to a bank, where they begin to fight among themselves as a result of one of the Wizard's spells. During the fracas, Medusa kidnaps Herbie. After the spell has worn off, the Wizard challenges the Fantastic Four to rescue their comrade from his mansion. At the Wizard's mansion, they have to survive the many traps and liberate their friend as well as ensuring that the Frightful Four are defeated for good.

On board a commercial airline, the Fantastic Four are travelling to Gateway University in St. Louis where Reed is due to receive and honorary degree when the plane is hit by a mysterious flying object. Together, the Four save the plane and then travel to the university. Reed meets with the dean, Johnson and tells him of what happened with their plane, and that he saw the object fly back to the university. Johnson suspects what has happened and shows Reed to his old teacher, Professor Gilbert's laboratory. There Gilbert shows his former student his latest creation, a giant winged android called Dragon Man. He reveals that Dragon Man's test flight was responsible for accidentally striking the plane earlier. With the help of his assistant, George, Professor Gilbert demonstrates how Dragon Man works by using a thought helmet to control the android. Resentful of not receiving what he thinks is his due, Gilbert's assistant George takes control of the Dragon Man, and disrupts Reed's ceremony. Gilbert tells Reed that the only thing Dragon Man is vulnerable to is extreme cold, and so the Fantastic Four formulate a plan to stop the Dragon Man once and for all.

A strange green rocket lands on Earth and turns into a powerful green alien. The first people he meets up with are a gangster named Grogan, and his accomplices who have just robbed a bank. The alien tells them that he is from the planet Popup on vacation and has no name. Since he can do just about anything, Grogan dubs him the Impossible Man. Taking advantage of his ignorance of Earth culture, Grogan plans to use Impossible Man to rob every bank in New York. The next day, they successfully steal a large amount of cash from a city bank. The Fantastic Four are alerted to the super-powered theft by the police, and track Impossible Man and his acquaintances to a local restaurant. Completely fooled by Grogan, the Impossible Man refuses to listen to the Fantastic Four's explanation as to what is really going on. Still not wanting to listen, the Impossible Man does battle with the Fantastic Four and manages to hold them off. Knowing that sooner or later the Four will have proof that Grogan is directly responsible for the resulting bank thefts, he has the Impossible Man go to the next bank on his own. At the bank, the Fantastic Four disguise themselves as bank workers and convince the Impossible Man to find out what Grogan really thinks about him. They have to hope that the Impossible Man listens to their advice and turns against Grogan, as well as figuring out a way of getting him off the Earth before he causes any more trouble.

Doctor Doom leaves Latveria for New York on his royal rocket. Herbie detects the rocket's launch and Doom makes his way to the Latverian embassy. There, he broadcasts a holographic message to the Fantastic Four. Doom tells them that unlike previous occasions, this time he will finally defeat the Four. Not wanting Doom to make the first move, the Four split up and search the city for Doom's whereabouts. Having no success, Reed suggest the team take time out to relax. Ben goes bowling, Reed attends a science convention, and Sue gets here hair done. At each of their chosen locations, the Four are constantly followed by bizarre Doom-balloons floating in the air. The balloons cause the Four to attract ridicule and disrespect from various members of the public, and so they return dejected to the Baxter Building. Doom again contacts the Four and tells them that they will never be able to stop him. Reed discovers that each of them was bugged by Doom, explaining how the balloons followed them. Meanwhile, after having his request for total command of the U.S. armed forces denied by the Pentagon, Doom cuts off the electrical power for the whole country. As the nation grinds to a halt, Reed and Herbie track down Doom's satellite-ship base, but as Doom's balloons were constantly recording the molecular structure of each of the Four, only Herbie with his interchangeable components can disarm Doom's alarm system. After doing so, the rest of the team are able to enter Doom's ship and contend with many booby traps. In the end it falls to a battle of wits between Reed and Doom to determine the victor.

Reed's new experiment yields in the discovery of another dimension known as the Negative Zone. He wants to see if the Zone contains new power sources to help the Earth's energy crisis. Sue then shows the team Lockjaw, the Inhumans' giant teleporting dog, who she borrowed from Medusa. With Lockjaw's help, Ben and Sue are able to teleport themselves to the Negative Zone to retrieve mineral samples. While in the Zone, they come under attack by winged creatures, but manage to defeat them. They soon discover a very large cocoon and realize that the creatures were trying to keep them away from it. Sue hears a heartbeat coming from the cocoon and Reed tells here to bring it back with them. Feeling tired and overworked, the Four decide to go out for dinner leaving Lockjaw to guard the cocoon. Feeling rejected, Lockjaw looks to the cocoon for company. Soon the cocoon bursts open, revealing the evil Blastaar, capable of firing enormous power bolts from his hands. Having overheard the Four talk of Lockjaw's teleporting abilities, he uses the dog to take him all over New York, destroying anything in his way. The team attempt to stop him, but with Lockjaw's help, Blastaar always gets away. The Fantastic Four are left with having to track Blastaar down and imprison him again.