雷欧·奥特曼是圆谷特摄剧《雷欧奥特曼》的主人公。来自于狮子座·L77星云的炎之战士，因故乡被毁灭而在宇宙中流浪，来到地球后化身名为“凤源”的年轻人生活，将地球当做自己的第二故乡。在赛文·奥特曼倒下后，雷欧肩负起了保卫地球的使命，加入宇宙巡逻队MAC，和凶恶的怪兽、宇宙人战斗。起初的雷欧实力比较弱，但是在诸星团的严格指导下逐渐成长为一名奥特战士，战胜了一个又一个的强敌，是一名擅长宇宙拳法的高手。后成为奥特兄弟的一员，现为光之国宇宙警备队的成员之一，亦是赛文·奥特曼之子赛罗·奥特曼的恩师，是昭和系奥特曼中的格斗达人 。

本作講述了一名來自「獅子座L77星」的戰士超人力霸王雷歐的故事，由於其故鄉遭到馬格瑪星人（マグマ星人）完全摧毀，他因而來到地球，並化身為人間體「鳳源」在日本的城南體育中心擔任教練。在第1話中，彈則提及自從L77星毀滅後，在地球已經過了一個月的時間。

與此同時，超人七號在太郎離開後再次承擔地球的防衛任務，並再次化身為人間體「諸星彈」，成為宇宙巡邏隊MAC的隊長。當馬格瑪星人帶領著雙子怪獸：紅吉拉斯、黑吉拉斯將侵略的魔手伸向地球時。在激烈的戰鬥中，彈的右腳因重傷並失去了變身能力，而源＝雷歐則代替他的決心以保護第二個故鄉「地球」加入了MAC。作為一名未成熟的戰士，源在接受彈的特訓的同時，也將持續與其他的外星人和怪獸們戰鬥。

Ultraman Leo is an Ultra foreign to the Land of Light, instead hailing from the fallen Planet L77 of the Leo constellation, where he and his brother Astra were royalty. After his planet was destroyed by Alien Magma and his twin Giras kaiju, Leo traveled to Earth, intending to make it a second home. While there, he blended into human society as Gen Ohtori, and became a gym instructor at a fitness club for children. But, when the Alien Magma that destroyed L77 came to conquer Earth, he was forced to take action. Leo met Ultraseven during his initial fight against Alien Magma, and became the first Ultra Crusader of Earth to not hail from the Land of Light. Unlike most of the previous Ultras to visit Earth, Leo's fighting style specializes in martial arts giving him far greater physical abilities than any other known Ultras. Despite his foreign origins, he, along with Astra, were readily accepted into the pantheon of the Ultra Brothers. During his tenure on Earth, Leo and Seven became very close, and it was for this reason that Seven would eventually entrust the training of his son Ultraman Zero to Leo and Astra.

ウルトラマンレオは故郷のL77星をマグマ星人たちに滅ぼされ、地球へやって来た。レオは、故郷によく似た美しい地球を守ることを決意する。レオの人間体であるおおとりゲンはMACに入隊し、モロボシ・ダン隊長と共に怪獣たちに立ち向かう。生きる厳しさと哀しさを鮮烈に謳う――ウルトラマンレオの成長の物語！

Depois de ter sido brutalmente mutilado por alienígenas do mal, Dan Moroboshi (Ultra Seven) leva sob sua asa um feroz Ultra-ser ainda jovem de L-77, e batizá-lo de "Ultraman Leo," para continuar sua missão de defender a Terra.