The Mysterious Dinosaur Base (謎の恐竜基地 Nazo no Kyōryū Kichi) is the 10th episode of Ultraman. On the DVD release it is only called Mysterious Monster Base.
A scientist named Nakamura was giving his animals food and they didn't like it. The scientist went to feed his pet Jirahs. He said if Jirahs is here, then there will be more fish here, but he had to be hidden. The next day, people were fishing in Lake Kitayama and they got lots of fish. The Science Patrol went to investigate the lake of sudden increase in fish numbers. They went with the Jet VTOL equpped with S16 submarine. Arashi had to be in the submarine. When they got there Arashi had to put on many things like the sonar and the video recorder. After the investigation was done Cap Muramatsu told them they could have a "Special" Vacation.
The press wanted to get information about the Loch Ness Monster so they asked Nakamura. When the press was there, he told them that it was probably the Diploducus. Then they got out. The male reporter asked where the female one was, and Hayata said she was with Ide night fishing. Ide and the female reporter were night fishing and they saw Nakamura rowing back to shore. Secretly, they were following him and Ide put up a sign on the tree. After they followed him, they got trapped In his own lab. Ide had his Science Patrol communicator broken so he couldn't call anyone.
A few fishermen put in poison in the lake, and they got bunch of fish. However Jirahs himself awakened and rose out of the water, revealing to be a giant frilled dinosaur-like monster. The Science Patrol used SPIDER on Jirahs but Nakamura stopped Arashi from using the SPIDER. Ide got his transmitter working again and he told them he was trapped. Ripping off his "face", Nakamura reveals himself as Professor Nikaido. The other Science Patrol members but Arashi, went to rescue Ide. Fleeing from the Science Patrol, Nikaido ran to Jirass and orders him to rampage and destroy everything, but he got stomped on accidentally, leaving the monster without someone to control him. Arashi then continued using his SPIDER gun, until it ran out of energy. The Science Patrol rescued Ide and the female reporter and vacated the lab. Hayata went out of sight of the others to use the Beta Capsule, becoming Ultraman.
The ruthless Jirahs was about to demolish the lab, when Ultraman stopped him in a nick of time. As the two giants prepared to fight, Jirahs demonstrated his might by throwing a rock in the air and hitting it with his heat ray. Ultraman did the same: but with one hit from his Specium Ray, he destroyed two rocks. Jirahs got angry at Ultraman's greater ability and charged at him, but Ultraman pushed him away. When the monster tried again, the spaceman shoved him again with a mighty force that knocked Jirass down to the ground. As Ultraman laughed at him, Jirahs got up and fired his heat ray, but he dodged the beam three times. Ultraman then ripped off Jirahs' frill wrapped around his neck, using it as a red flag like to imitate a bull fight, taunting the monster. Tossing away the frill, Ultraman wrestled Jirahs, throwing him to the ground in a near judo-style and pouncing on him, making both spaceman and monster alike roll. At the same time, the light of Ultraman's Color Timer changed from blue to red and began to blink when Jirass throws him off. Ultraman and Jirahs then charged at each other, but the tables were turned when Ultraman jabbed him with the Ultra Chop. Jirass bled out, and dropped to the ground dead. Instead of destroying Jirahs' body with another Specium Ray, Ultraman gave respect to the monster by placing his frill back on his neck. With his Color Timer still flashing its warning signal of losing solar energy while on Earth, he flew away. The injured Professor Nikaido was crawling on the ground, and looked up at the dead Jirahs. He cried out for his creation before he died as well. That night, Jirahs' body was cremated to rest in peace.
- Toshio Muramatsu (ムラマツ・トシオ Muramatsu Toshio): Akiji Kobayashi (小林 昭二 Kobayashi Akiji)
- Shin Hayata (ハヤタ シン Hayata Shin): Susumu Kurobe (黒部 進 Kurobe Susumu)
- Akiko Fuji (フジ アキコ Fuji Akiko): Hiroko Sakurai (櫻井 浩子 Sakurai Hiroko)
- Daisuke Arashi (アラシ・ダイスケ Arashi Daisuke): Sandayū Dokumamushi (毒蝮 三太夫 Dokumamushi Sandayū)
- Mitsuhiro Ide (イデ・ミツヒロ Ide Mitsuhiro): Masanari Nihei (二平 政成 Nihei Masanari)
- Ultraman (ウルトラマン Urutoraman): Masao Nakasone (中曽根雅夫 Nakasone Masao) (grunts)
- Narrator (ナレーター Nareta): Koji Ishizaka (石坂浩二 Ishizaka Koji)
- Ultraman Volume 3 features episodes 9-12.
- The US DVD Release Ultraman Here He Comes From the Sky features episodes 1-10.
- The Jirahs suit itself, is actually a modified 1964 Godzillla suit, with the exception of the head, which is actually the head of the 1965 Godzilla suit. (Since the 1964 suit was used for Gomess and the head was covered in hair and long spikes. Was cutoff and replaced with the G65 head and the frill was also attached to the suit, and thus creating Jirahs.)
- This episode marks the time Ultraman yells "Shuwatch!", usually when he flies in the later episodes of the series. He shouts his battle cry as he shoves Jirahs in their fight. In the English dub, Ultraman was silent for nearly a minute as he and Jirahs begin to fight. But after he gave Jirahs the first push, his grunts and yells returns in their resuming fight. After he killed Jirahs and returned his frill to his corpse, Ultraman did not yell "Shuwatch!" as he fly away.