(First Aired: August 14, 1966)
Screenplay by: Keisuke Fujikawa
Directed by: Toshihiro Iijima
The reporter Kobayashi and the geologist Dr. Matsuo are found dead by the police. Both bodies have been smothered to death by something, so Science Patrol is called in to look. At the same time, Science Patrol is looking into the murder of the genetic botanist Dr. Yamada. So, Muramatsu sends Ide and Arashi to Dr. Yamadas place, noting that Yamadas death was similar to the two new cases, which he and Hayata will investigate. Hayata and Muramatsu first check out Kobayashis death, which occurred under an overpass at midnight near his crashed car. The car had slipped on a sort of green substance. His bodys location is a little bit further from the car, indicating it wasnt the crash that killed him. Its also surrounded by green substance, and the car is stained in it too.
Meanwhile, Arashi and Ide are at Dr. Yamadas botanical laboratory-greenhouse the site of his death. Inside are all sorts of genetic marvels created by Dr. Yamada like super-sized vegetables. His assistant Miss. Maki, who now runs the lab, explains that he had returned from an expedition to Oirisu Island not long before he died, and he returned with a beautiful flower he wished to genetically improve on. She shows them a hole in the greenhouse soil, explaining that Dr. Yamadas corpse was found next to it. It was where the flower he found was planted, but when Yamada was found, the flower was missing. Arashi reports this to Fuji, so they can get a list of those who went on the Oirisu Island expedition. The expedition group was made of Kobayashi, Dr. Matsuo, Dr. Yamada, a zoologist named Dr. Sakai, and a photographer named Setsuko Hamaguchi.
Back with Hayata and Muramatsu, the two are visiting Dr. Matsuos death scene. With them is Dr. Iwamoto, Science Patrol's most frequent expert for scientific consultation. Iwamoto explains that the green slimy stuff is also at Matsuos death. His analysis indicates its something akin to the trails left by snails, though he notes the green coloring indicates chlorophyll, making it unlikely that the killer was an oversized gastropod. Then problems pile up when Fuji reports that Dr. Sakai has been found dead as well! Hayata, Muramatsu, and Iwamoto head to Sakais crime scene. Ide and Arashi join them moments later, where Iwamoto explains that studying the slime at the scene via Dr. Sakais microscopes reveals the green slime is indeed chlorophyll radioactive chlorophyll to be exact. Arashi informs him of the disappearance of the Oirisu Island plant, showing Iwamoto a picture given to them by Miss. Maki. Iwamoto identifies the lovely plant as a Miroganda Flower while Ide notes that Yamada often used radiation in his genetic experiments.
The team soon returns to HQ, where Ide suggests that Hamaguchi the photographer might have murdered the other expedition members. So, Muramatsu, Arashi, Ide, and Hayata go to her place and ask her a few questions. Hamaguchi tells them she has no idea why the rest of the expedition was killed, but it is highly likely she will be killed next. Muramatsu asks her if anything strange happened while on the Oirisu island expedition, to which Hamaguchi says she found the Miroganda Flower that Yamada took back by a stream. Further down the river, she briefly got her foot stuck in the maw of a man-sized carnivorous plant resembling a grotesquely deformed flower. Upon hearing this, Hayata hatches a theory: the ugly carnivorous plant that attacked her was the juvenile form of a Miroganda Flower, like how the Ugly Duckling became a beautiful swan. Dr. Yamadas radiation experiments had mutated the Miroganda Flower into its juvenile form and gave it the ability to walk, as well as the intelligence to it to attack him and other expedition members. Now, Science Patrol needs to protect Hamaguchi from the leafy murderer.
Science Patrol spends the entire night at Hamaguchis place, but theres no sign of the killer plant. Arashi encounters the killer the next morning, and it is the hideously misshapen carnivorous plant that Hamaguchi encountered, if it had grown a slug-like foot to walk upright. The creature is no longer a Miroganda Flower, but the monster Greenmons. Arashi wrestles the plant monster, but has to contend with a spray of poison gas from its central orifice. The team overhears Arashis cries for help and rushes in. They fire at Greenmons with their ray guns, eventually forcing it to fall into the nearby river, where it flows down with the stream.
The team return home with a sense of victory, but Dr. Iwamoto delivers dreadful news. He believes Greenmons is not dead, but the radioactive beams of their ray guns have caused the monster to grow even bigger. Then, Science Patrol is called to head to Marunouchi and get rid of a monster. Sure enough, the police and the JSDF are everywhere and ready to fire. The monster enters, revealing that it is indeed what Dr. Iwamoto has feared Greenmons is now taller than the buildings around it! Arashi is the first to shoot, but Greenmons suffocates him with a blast of poison gas. Hayata grabs him and takes him to safety, where he places the unconscious Arashi aside and becomes Ultraman! Ultraman and Greenmons face off, and the hero swiftly ends Greenmons life with a Specium Ray that sets the plant monster alight, turning it into a smoldering heap in minutes. With that, Ultraman leaves and Science Patrol does likewise.