Pursue the Mystery of the Transformation Terrible-Monster (変身超獣の謎を追え Henshin Chōjū no Nazo o Oe!) is the sixth episode of Ultraman Ace.
An astronaut named Koyama returns to Earth to his wife and son, but is waylaid by Yapool and his latest creation, Brocken. TAC helps Koyama bring his spaceship in safely, not realizing that he has been possessed by Brocken. Koyama goes home to his son, who notices that his father hasn't removed his gloves.
- Seiji Hokuto (北斗星司 Hokuto Seiji Hokuto Seiji): Keiji Takamine (高峰 圭二 Takami)
- Yuko Minami (南夕子 Minami Yūko): Mitsuko Hoshi (星光子 Hoshi Mitsuko)
- Dan Umezu (梅酢ダン Umezu Dan): Umezu Akinori (梅酢秋則 Akinori Umezu)
- Goro Ryu (竜五 郎 Ryu Goro): Tetsurō Sagawa (瑳川哲朗 Sagawa Tetsurō)
- Ichiro Yamanaka (山中一郎 Yamanaka Ichiro): Okita Shunichi (沖田駿一 Shunichi Okita)
- Tsutomu Konno (今野勉 Konno Tsutomu): Masaaki Yamamoto (山本正明 Yamamoto Masaaki)
- Noriko Mikawa (美川 のり子 Mikawa Noriko) Keiko Nishi (西恵子 Nishi Keiko)
- Kozo Yoshimura (吉村公三 Yoshimura Kozo): Koyo Sano (佐野光洋 Sano Koyo)
- Kaji Yoichi (梶洋一 Yoichi Kaji): Katsumi Nakaiyama (中山 勝己 Nakaiyama Katsumi)
- Ultraman Ace (ウルトラマンエース Urutoraman Ēsu Voice): Gorō Naya (納谷悟朗 Naya Gorō)
- Yapool (ヤープル Yapurū Voice): Hirofumi Takada (高田裕史 Takada Hirofumi)
- Narrator (ナレーター Nareta): Shin Kishida (岸田森 Kishida Shin)
- Akira Koyama (小山彰 Koyama Akira): Akiji Kobayashi (小林 昭二 Kobayashi Akiji)
- Ultraman Ace (ウルトラマンエース Urutoraman Ēsu) Tadashi Nakanishi (中西正 Nakanishi Tadashi)
- Brocken ((ブロッケン Burokken): Toru Kawai (河合 徹 Kawai Tōru)
- Ultraman Ace
- Ultraman (Stock Footage)
- Zoffy (Stock Footage)
- Ultraseven (Stock Footage)
- Ultraman Jack (Stock Footage)
- Ultraman Ace Volume 2 features episodes 5-8.
- Akiji Kobayashi who was playing Akira Koyama previously played Toshio Muramatsu in the Original Ultraman Series. He was now currently playing one of the main characters of Kamen Rider, Tōbei Tachibana. Also, the fifty-ninth episode Kamen Rider, The Bottomless Swamp Monster, Earthworm Man! aired on NET a day after the airing of Ultraman Ace's episode on the Tokyo Broadcasting System.
- Stock footage of "Shine! The Five Ultra Brothers" when the four previous Ultra Brothers did their poses was used in this episode in the close-up of Ultraman Ace's Color Timer.