(First Aired: July 31, 1966)
Screenplay by: Masahiro Yamada
Directed by: Toshihiro Iijima
Fuji and Hoshino are at a feudal castle looking over a well that has gained a reputation for the menacing rumbles coming from its depths. According to their tour guide, the well has a secret pathway for reaching the seacoast if the castle came under siege, but the construction of a hydroelectric power plant has prevented archaeologists from entering the caves for study. The ever-adventurous Hoshino enters the well himself, despite Fuji and their tour guide’s protests. Inside, Hoshino notes a bit of wind, which he guesses is coming from the other side of the cave. What he finds instead is an enormous eye blinking at him! Meanwhile, Fuji enters the well after Hoshino, but the ground begins to shake violently. The earth soon gives way as a bellowing roar rings over the region, but nothing exits the hole. As people flee the area, the earth sinks, trees splinter, and the power plant takes damage as if being hit by an invisible force. A ghostly shape briefly appears and attacks the power plant's cables before vanishing.
Underground, Fuji and Hoshino have survived and found the other side of the caverns, which happens to be a sea cave. Back at HQ, Muramatsu, Hayata, and Arashi are preparing to head out to investigate the report of the power plant’s destruction and find Fuji and Hoshino. They land by the power plant’s wreckage and speak to a plant worker, who tells them he saw a glimpse of a monster. He reports that the monster vanished in thin air after it ate the plant’s electricity. Fuji and Hoshino are located afterwards, both having swam to the beach from the sea cave, so the men decide to get some information from them. They report hearing the monster but not seeing it, leading the team to conclude their enemy is invisible. They then receive a report that the nearby Iwa Power Plant has lost all its electrical output.
Hayata and Arashi make their way to the Iwa Power Plant, where workers report there was no way the perfectly functioning power plant could have suddenly lost all its electricity. Then the nearby cables begin to break and a roar rings in the air. The culprit materializes, revealing itself to be Neronga, a four-legged rhinoceros-like reptile with yellow scales, a barrel-shaped body, and two antennae. Arashi’s shots are repaid by a bolt of lightning from the monster’s nasal horn that knocks him out. Then, it begins to “eat” the electricity by sucking up the voltage through its horn and antennae. Neronga finishes its meal and leaves, vanishing in thin air as it wanders away.
Back at HQ, Science Patrol figures that Neronga will be heading for a thermal plant at the outskirts of Iwa City next. The problem is that power plant supplies 60% of the Kanto region’s electricity, including Tokyo! Hoshino then enters with some research and conclusions he made on Neronga. Neronga was a creature that had been defeated long ago by a samurai and sealed underneath the well. The power plant’s construction of underground cables had fed the imprisoned monster, rejuvenating its strength and ballooning it to a vast size. Neronga had also learned to manipulate electricity over the years spent underneath the power plant, hence its need to consume electricity, which it needs to become invisible. With some information at hand, Science Patrol calls the JSDF and moves out to defend the power plant. However, nobody notices Hoshino take Arashi’s ray gun or sneak aboard the Jet V-TOL to join the team against Neronga.
While Hoshino sneaks around the power plant, Arashi and Fuji are looking for him as they’ve finally noticed the missing weapon. That’s when Neronga’s thunderous bellow is heard, signaling Science Patrol and the JSDF to be on their guard. Neronga materializes shortly after a couple lucky shots and begins to demolish the power plant, oblivious to the barrage of firepower against it. Hoshino follows at a safe distance, eventually getting in front of Neronga and firing at its face. Science Patrol spots the boy, who has impressively managed to blow out one of Neronga’s eyes. Unfortunately, Hoshino’s stolen ray gun runs out of energy, and when they boy attempts to flee, he trips. Neronga proceeds to lumber forward to destroy the rest of the power plant and squash Hoshino. Hayata runs in with Arashi to save Hoshino, who weakly begs for Ultraman to come save the day. Hayata instructs Arashi to take Hoshino to safety while he heads in. Once far enough, Hayata becomes Ultraman! Neronga proves to be no match for Ultraman, who snaps the monster’s nasal horn before killing it with a Specium Ray. Ultraman leaves afterwards and Hayata rejoins the group, who fly back to HQ soon after.
