The Invisible Challenger (姿なき挑戦者 Sugata Naki Chōsen-Sha) is the first episode of Ultraseven.
Aliens from the planet Cool have kidnapped a number of humans for experimental purposes leading to an impending invasion of the planet Earth. The Terrestial Defense Force is sent to investigate and meets a mysterious wanderer named Dan Moroboshi who helps destroy the invading alien forces. No one knows his true identity, but Moroboshi transforms into Ultraseven, a silver and red superhero from another galaxy committed to protecting the Earth from monsters and alien threats.
The UG is ordered to investigate several mysterious disappearances, which are suspected to be the work of aliens. They are called together, and the captain, Kaoru Kiriyama, finds that two defence force members have also been mysteriously abducted. He takes this as a challenge by their enemies. Suspecting the aliens to be aware of their movements, he warns everyone to be on guard and notes that the enemy may be nearby.
Shigeru Furuhashi and Soga are deployed, while Anne Yuri and Amagi stay to study the actions of the enemy. While on the road, the former are stopped by a strange man, who tells them not to progress any farther. A police officer drives by and says nothing suspicious has been reported, and continues on despite Dan's warnings. His car vanishes.
Dan tells the two members of the Ultra Guard that the Cool Aliens have been gathering human specimens and experimenting on them for years, and now intend to move into their plan for Earth domination and destroy the Ultra Guard. Just then they are interrupted by bolts from the sky, and the two officers are wounded. They flee, with Dan at the helm, and return to base.
They report that they were attacked by an invisible spaceship. Just then, one of their screens is hacked and they are contacted by an Alien Cool. It tells them to surrender the Earth, or else the captive humans will be destroyed. After the leaders refuse to comply, the aliens begin their assault on Earth. At this direct assault, the leaders are completely unsure of how to proceed.
Anne Yuri suspects Dan might know something, and asks him how to defeat the aliens. He suggests that they spray the spaceship with paint to render it visible. Scientists work on making a device which which to do it, as radar tracks down the ship itself. When the device is ready, the Ultra Hawk No.1 departs with the team (as well as Dan) inside it. They manage to make the ship visible, and battle it. The aliens retreat, leading the Ultra Hawk to a rocky area where they manage to cause it to crash land.
- Dan Moroboshi/Ultraseven (モロボシ・ダン／ウルトラセブン（声） Moroboshi Dan/Urutorasebun): Kohji Moritsugu (森次 晃嗣 Moritsugu Kōji)
- Anne Yuri (友里アンヌ Yuri Annu): Yuriko Hishimi (ひし美 ゆり子 Hishimi Yuriko)
- Shigeru Furuhashi (フルハシ・シゲル隊員 Furuhashi Shigeru): Sandayū Dokumamushi (毒蝮 三太夫 Dokumamushi Sandayū)
- Kaoru Kiriyama (キリヤマ・カオル Kiriyama Kaoru): Shoji Nakayama (中山昭二 Nakayama Shōji)
- Soga (ソガ Soga): Shinsuke Achiha (阿知波信介 Achiha Shinuske)
- Amagi (アマギ Amagi): Bin Furuya (古谷敏 Furuya Bin)
- Alien Cool (クール星人 Kūru Seijin Voice): Koji Yada (矢田耕司 Yada Kōji)
- Narrator (ナレーター Narētā): Hikaru Urano (浦野光 Urano Hikaru)
- Ultraseven (ウルトラセブン Urutorasebun): Koji Uenishi (上西弘次 Hirotsugu Uenishi)
- Windom (ウインダム Uimdamu): Sadao Sunohara (春原貞雄 Sunohara Sadao)
- Ultraseven Volume 1 features Episodes 1-4.
- This episode was later featured in Ultraman Retsuden episode 85 "The Invisible Challenger! Ultraseven Appears!!."