"Who was that voice? We didn't know. But it goes to show that sometimes, strange things beyond what you can understand can help guide you. I bet that's part of why I'm Ultraman."
- ―Riku's narration at the end of the episode.
The Sword of an Oath (誓いの剣 Chikai no Tsurugi) is the ninth episode of Ultraman Geed.
Leito's daughter Mayu is affected by a Little Star that lets her teleport. It's up to Geed and Zero to protect her, while Laiha confronts Fukuide Kei.
Leito's daughter Mayu develops signs of Little Star lately, as well as the ability to teleport. Concerning with Kei's approach, Riku decided to fight against the Little Star targets while Laiha and Leito would find and attack Kei in a nearby area. AIB as well as voices their concern for Kei as he lacks a record on Earth and an actual home address by his editor. Initially scared in the Nebula House, Laiha's words managed to convince Mayu to stay and accompany Pega as protection from monster attacks.
All of a sudden, a monster tries to burrow the Nebula House in search for Mayu. Moa searched the very first monster attack in Japan, recorded on Mt. Mitsuse 6 years prior and discovered Laiha to be the sole survivor. Geed transforms and attacked the monster but Kei is nowhere to be found nearby them. Instead, Skull Gomora reappeared in Mt. Mitsuse, forcing Geed to leave the aforementioned monster to Zero. Hearing that Laiha is after Skull Gomora's location, Mayu quickly teleports away. After finishing the previous monster, Zero/Leito raced to Mt. Mitsuse to Mayu's side, who failed to stop Laiha when Geed had just defeated Skull Gomora.
After receiving the Zero Capsule from Maya's Little Star, Riku went to Mayu as he discovered Laiha's intention: Skull Gomora was the same monster that attacked Mt. Mitsuse 6 years prior, killing everyone including her parents and left her as the sole survivor. Laiha approaches Kei, remembering him as the true identity of said monster whose attack was to harvest her Little Star before it vanished. The two engaged in a fight and Laiha was about to kill him until "something" held her back. Kei took this opportunity to leave, allowing Zero to figure his identity as an Alien Sturm while AIB watches from afar.
Back at the Nebula House, Laiha recalled that she was stopped by a mysterious voice who proclaimed her actions as irrational. Riku simply smiles since she complied to those words.
- Riku: "Ultra Capsule Navi! Here are today's capsules!"
- Riser: Ultraman Ginga!
- RE.M.: Ultraman Ginga is from the future. Height: microscopic to infinite. Weight: zero to infinite. Notable for his Ginga Crystals, which shine with rainbow colors.
- Riku: Next up is this one!
- Riser: Tyrant!
- RE.M.: The despotic monster, Tyrant. Height: 62 meters. Weight: 57,000 tons. Created by combining the strengths of assorted other monsters.
- Riku: Be sure to watch next time!
- Riku Asakura (朝倉リク Asakura Riku)/Ultraman Geed (voice): Tatsuomi Hamada (濱田龍臣 Hamada Tatsuomi)
- Laiha Toba (鳥羽ライハ Toba Raiha): Chihiro Yamamoto (山本千尋 Yamamoto Chihiro)
- Moa Aizaki (愛崎モア Aizaki Moa): Mayu Hasegawa (長谷川眞優 Hasegawa Mayu)
- Leito Igaguri (伊賀栗レイト Igaguri Reito): Yuta Ozawa (小澤雄太 Ozawa Yūta)
- Kei Fukuide (伏井出ケイ Fukuide Kei): Kunito Watanabe (渡辺邦斗 Watanabe Kunito)
- Zena (ゼナ Zena): Hideyoshi Iwata (岩田栄慶 Iwata Hideyoshi)
- Mayu Igaguri (伊賀栗マユ Igaguri Mayu): Mirei Shimizu (清水美怜 Shimizu Mirei)
- Alien Pegassa Pega (ペガッサ星人ペガ Pegassa Seijin Pega Voice): Megumi Han (潘 めぐみ Han Megumi)
- RE.M. (レム Remu Voice): Suzuko Mimori (三森すずこ Mimori Suzuko)
- Ultraman Zero (ウルトラマンゼロ Urutoraman Zero Voice) : Mamoru Miyano (宮野真守 Miyano Mamoru)
- Alien Shadow Zena (シャドー星人ゼナ Voice): Shintaro Asanuma (浅沼晋太郎 Asanuma Shintarō)
- Suzumi Toba (鳥羽スズミ Toba Suzumi): Kokoro Hiho (妃鳳こころ Hihō Kokoro)
- Danji Toba (鳥羽ダンジ Toba Danji): Megumi Uemura (植村恵 Uemura Megumi)
- Ultraman Geed: Hideyoshi Iwata (岩田栄慶 Iwata Hideyoshi)
- Ultraman Zero: Akira Okabe (岡部暁 Okabe Akira)
- Pega: Satomi Maruta (丸田聡美 Maruta Satomi)
- Tyrant: Yoshiki Kuwahara (桑原義樹 Kuwahara Yoshiki)
- Skull Gomora: Kenji Kajikawa (梶川賢司 Kajikawa Kenji)
- Ultraman Geed
- Solid Burning
- Ultraman Zero
- Laiha's Ring tone is Tiga's Color Timer running out of energy.
- Tyrant shooting his hook re-uses Alien Magma's sound effects.
- This episode establishes Skull Gomora as the first monster to attack Japan in the World of Side Space.
- In the Indonesian dub that aired on RTV, this episode was named "The Secret of Laiha's Sword".
- In the article Moa is reading on the survivors of Skull Gomora's initial attack, one of them is Jiro Satsuma, the mountain climber whom Ultraseven saved and took the appearance of to become Dan Moroboshi.