The Rainbow's Egg (虹の卵 Niji no Tamago) is the eighteenth episode of Ultra Q.
Jun, Ippei and Yuriko investigate a hillside accident involving a truck that was transporting a uranium capsule. After rescuing the drivers, they learn that the monster Pagos had surfaced from underground, following the appearance of a mysterious rainbow, and caused the accident. When the rainbow appears again, Pagos rises and terrorizes a group of children that plays in the area. A young girl named Piko has discovered the uranium capsule, which she percieves to be the magical "rainbow's egg" that can grant wishes of healing. Piko has a particular fondness for an elderly woman named Tamiko, whom the children call "grandmother", and wants to use the egg's magic to help her walk again. Attracted to uranium, Pagos initially approaches the young girl as she struggles with her quarry, but is ultimately lured to a nearby atomic power plant instead. The monster attacks and devastates the plant before neo-neutron missiles can be launched against it. Pagos is drenched by the radioactive fallout from the exploding missiles and solidifies into rock. Crashing to the ground, the monster shatters apart and the mysterious rainbow disappears. Piko, however, is determined to present the "egg" to Tamiko and is reluctant to surrender it to Jun and the others. When Tamiko arrives, she is so overjoyed to find that Piko is safe, she rises from her wheelchair and miraculously begins to walk again!
- Jun Manjome (万城目淳 Manjome Jun): Kenji Sahara (佐原 健二 Sahara Kenji)
- Yuriko Edogawa (江戸川由利子 Edogawa Yuriko): Hiroko Sakurai (桜井 浩子 Sakurai Hiroko)
- Ippei Togawa (戸川一平 Togawa Ippei): Yasuhiko Saijo (西條 康彦 Saijo Yasuhiko)
- Narrator (ナレーター Nareta): Koji Ishizaka (石坂 浩二 Ishizaka Koji)
- Professor Itoigawa (糸魚川博士 Itoigawa-hakase): Hideo Kidokoro (城所 英夫 Kidokoro Hideo)
- Truck driver (トラック運転手 Torakku untenshu): Youichi Miyagawa (宮川 洋一 Miyagawa Youichi)
- Nuclear power plant director (原子力発電所・所長 Genshiryokuhatsudensho shocho): Zenpei Saga (嵯峨 善兵 Saga Zenpei)
- grandmother (お婆ちゃん obachan): Fukami Harue (春江 ふかみ Harue Fukami)
- piko (批伊子 piko): Hikaru Shirakawa (白川 ひかる Shirakawa Hikaru)
- Truck driver's Assistant (トラック運転手の助手 Torakku untenshu no joshu): Kiichi Hoshi (星 紀市 Hoshi Kiichi)
- bunta (ブン太 bunta): Soujiro Uchino (内野 惣次郎 Uchino Soujiro)
- Aochin (青チン Aochin): Hisanobu Ichikawa (市川 久伸 Ichikawa Hisanobu)
- Ki-bo (キー坊 Ki-bo): Takashi Okubo (大久保 隆司 Okubo Takashi)
- Tsuyoshi (強 Tsuyoshi): Kazunari Ehara (江原 一成 Ehara Kazunari)
- Nonko (のん子 Nonko): Sumiko Nakahara (中原 純子 Nakahara Sumiko)
- Midori (ミドリ Midori): Itsuko Gouda (郷田 いつ子 Gouda Itsuko)
- little (チビ chibi): Toru Toriumi (鳥海 透 Toriumi Toru)
- Tamiko (民子 Tamiko): Shino Shiba (柴 志乃 Shiba Shino)
Ultra Q Volume 5 Features episodes 17-20, & Total Natural Color DVD Volume 6 Features episodes 18-21.
- Originally Gomess was planned to appear in the episode in Pagos' place instead, however due to Tsuburaya being forced to return the Godzilla suit back to Toho Co. LTD, Pagos was created in its place thanks to the Baragon suit being loaned by Toho. The Baragon suit was also later adapted for the Ultraman monsters Neronga and Gabora.
- At the 18:06 mark, the puppetry wires on Pagos can be seen.