The Secret of the Lake (湖のひみつ Mizūmi No Himitsu) is the third episode of Ultraseven.
An unidentified object has landed somewhere near the Kiso Mountains. Dan and Furuhashi are sent to find any wreckage, or signs of life. They find a young girl swimming in Lake Azuma as well as a spaceship nearby. Both men go aboard the craft where they find a girl that looks exactly like the one they saw swimming in the lake. The two men are temporarily knocked unconscious by gas emitting from the walls of the ship. When Dan comes to, he realizes the Ultra Eye is missing. Without it, he can't turn into Ultraseven.
Not long after, a giant monster named Eleking emerges from Lake Azuma. Till he can locate the Ultra Eye, Dan uses one of his capsule monsters to send Miclas to battle the amphibious Eleking, a beast under the control of two insectoid twin aliens know as Alien Pitts posing as little girls with plans to wipe out all of mankind.
- 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)
- 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)
- Ultraseven Volume 1 features Episodes 1-4.
- This episode was later featured in Ultraman Retsuden episode 27 "The Secret of the Lake! Seven vs. Eleking!!"
- The Ultraman Orb episode, "Monstrous Waters", is a tribute to this episode.