Mefilas, an alien, wanted to possess Earth, because the primary star of his home planet is dying. But he goes about doing this the wrong way, by kidnapping Akiko Fuji and Isamu Hoshino. He is hoping to convince them to agree to his terms, since Mefilas dislikes violence. Unfortunately for him, they refuse to agree. Shin Hayata has already confronted Mefilas, and has exposed the Beta Capsule, his transformation artifact. But Mefilas paralyzes him before he can depress its ignition button with his right thumb.
Mefilas creates a surrogate of Fuji and enlarges her to gigantic size. As a further demonstration of his power, he summons illusions of Alien Baltan, Alien Zarab, and Kemur. Eventually, the Science Patrol finds and attacks the flying saucer which is Mefilas's starship. Mefilas retaliates, but he is forced to retreat. Science Patrol members rescue Fuji and Hoshino, leaving Hayata behind.
Taking a desperate gamble, Hayata falls down, causing the shock to jolt his right thumb into depressing the Beta Capsule's ignition button. Transformed, Ultraman faces off against Mefilas. The two, however, prove evenly matched. Mefilas stops their combat, saying that they have no need to fight against each other. Mefilas had wanted the hearts of Earth, but he sees now that he has lost to a child. He tells Ultraman that he will someday return, and vanishes.
- 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)
- Isamu Hoshino (ホシノ・イサム, Hoshino Isamu ): Akihide Tsuwaza (津沢 彰秀 Tsuzawa Akihide)
- Ultraman (ウルトラマン Urutoraman): Masao Nakasone (中曽根雅夫 Nakasone Masao) (grunts), Nisashi Kondo (近藤久 Kondo Nisashi) (voice)
- Alien Mefilas (メフィラス星人 Mefirasu Seijin): Seizo Kato (加藤精三 Katō Seizō) (uncredited)
- Ultraman (ウルトラマン Urutoraman): Bin Furuya (古谷 敏 Furuya Bin)
- Alien Mefilas (メフィラス星人 Mefirasu Seijin): Koji Ogi (扇 幸二 Ōgi Kōji)
"What's so funny, Ultraman?"
"Mefilas, your guess was completely wrong. No human would sell the Earth. Even a child like Satoru, wants to do good things for the Earth, and would never throw it away!"
"Shut up! What are you Ultraman? Are you an alien, or a human?"
"I am both. I was born to fight those who break universal laws, like you."
"Don't kid yourself. I will make this beautiful planet my own."
"Mefilas, I won't let you."
- ―Alien Mefilas I and Ultraman in Mefilas I's Space ship
"Alien Mefilas, go home to your own planet at once."
- ―Ultraman's and Alien Mefilas I's final words before their battle
"Let's stop. Ultraman, there's no use in two aliens fighting each other. I only wanted the heart of the Earth. But, I have lost. I even lost to a child. However this does not mean I have given up. There must be someone that will give me the Earth. They will come to me!"
- ―Alien Mefilas I ending his battle with Ultraman
- Ultraman Volume 9 features episodes 33-36.
- This episode was later featured in Ultraman Retsuden episode 84 "The Forbidden Word! Alien Mefilas's Challenge!!."