|This article is about a/an episode in Kamen Rider Blade.|
- Kazuma seals the Undead and sees Security Footage of Sakuya kidnapping Chief Karasuma.
- Shiori establishes her equipment at Kotarou's house and moves in there as well.
- They get alerted by the Undead Tracker which finds one at the Observatory where Amane is with friends
- Hajime learns of this also. After a tense battle, Hajime manages to save her
- Kazuma gets confronted by the monster again but is quickly intercepted by Hajime who seals the monster and then turns on Kazuma himself.
Blade continues to fight the monster. He scans his Tackle Card and attempts to hit the Undead but it springs out of the way and grabs him while flying. Kazuma scans his Tackle Card again and he and the Undead smash into the wall and both fall to the ground. The Undead's buckle un-clips and Blade seals it. Blade cancels the transformation and lies in exhaustion from the battle. Shiori comes to help him.
Later, both Shiori and Kazuma watch security camera footage of Chief Karasuma's office. Sakuya, as Garren comes into the room and attacks Karasuma and leaves with him slung over his shoulder. Shiori brands Sakuya as a traitor to which Kazuma disagrees on. He questions that maybe Sakuya had a reason but Shiori tells him that she doesn't want to hear a reason. Kazuma demands that Shiori tell him what BOARD exists for, what they are doing and where all the Undead are coming from. He asks her what he is fighting for... To be betrayed? The next morning, Kazuma sits outside Kotarou's house. Kotarou tells Shiori that that he has been sitting outside for ages and that whenever he tries to talk to him, he doesn't respond. He asks Shiori what happened to Chief Karasuma but she says doesn't really know. All she knows is that when she came back to BOARD, it was demolished. Kotarou asks her what BOARD is but Shiori doesn't know that either. She does tell him though, that she managed to move all the equipment from BOARD to the house. Kotarou accuses her of stealing but Shiori vindicates herself by calling herself a lab member and that she put a lot of research into the lab. She goes upstairs with her luggage while Kotarou berates her about the amount of it. Shiori reminds him of her privacy and goes into her room. Kotarou goes back outside to Shiori who asks him how many times he has been betrayed. Kotarou says that he doesn't keep count. Kenzaki tells Kotarou that each time it happened to him, he promised that he would never trust anyone again. However, he still trusted them. He calls himself naive to which Kotarou agrees with but labels it a positive trait of Kazuma's. Kotarou also tells Kazuma that he would prefer a person who is betrayed 100 times rather then a person who betrays 100 people. They call each other strange and are then alerted to Shiori who says that there is trouble. They race upstairs to find that the Undead Searcher is reacting. The map pops up to show an Undead in close proximity to where they currently are. The Undead manages to suffocate a number of people by using his vines and wrapping them around people's mouths. Kazuma appears hesitant at first and Shiori asks him why he isn't going as he's a Rider. She slaps him and asks him how long he will hesitate for and if he is okay with watching people suffer. She also tells him that it is a Rider's job to fight. He leaves, and rides off. At the Jacaranda, Hajime, senses the Undead without the help of a tracker and rushes off just as Kotarou's sister is informed about the Undead at the Observatory. She begins to worry as Amane, her daughter is there with her friends. Amane and one of her friends are hiding behind a pole from the monster. Hajime quickly runs out and rides off on his motorcycle. Kenzaki arrives at the observatory and hesitates again. He remembers back to his parents before also remembering Hirose's words. The Undead shoots it's vines out and wraps them around his neck. With Kazuma serving as the ultimate distraction, Amane and her Friend run away. The Undead starts to drag Kazuma in, but Kazuma manages to transform sending the monster flying away. They start to fight with the monster using his vines to leap to a quick advantage. Meanwhile, Hajime transforms into the Black Kamen Rider on the way. The Undead continues to dominate with its vines but after getting hit by Blade's Blay Rouzer, until it escapes. The Undead finds a hiding Amane who runs off in horror. The Undead begins to chase her and as she trips over, it goes for her. However, Chalice appears at the last second and knocks the Undead over with his motorcycle. Both Chalice and the Undead speak in a foreign Alien language to each other for beginning to fight. He easily dodges the Undead's Vines as they run around the Observatory with Amane following closely behind and watching. The Undead breaks a piece of Cement off with his vines and the remnants go flying towards Amane. Chalice realizes and quickly jumps in front of her, saving her, but taking pieces of cement in his back in the process. Chalice summons his weapon and knocks the Undead over before escaping with Amane. Kotarou, Amane's mother and Kenzaki search for Amane and are then met by Hajime who is carrying her in his arms. Kotarou and Kazuma both stand up and look at Hajime who tells them that he found her on the ground after falling over. Hajime walks off, ignoring questions from both Kazuma and Kotarou. Later, back at the Jacaranda Cafe, Kotarou shows Amane a picture of Kamen Rider Garren. She tells him that it wasn't that one that saved her and the one that saved her seemed to be talking to the Undead. Kotarou next shows her a picture of Blade but she doesn't recognize him either. Hajime sees the photo and reacts, getting up and walking to his room. Kotarou, Amane and her mother all brief Kazuma on who Hajime is. Kotarou tells him that half a year after his sister's husband died, he appeared looking for a room. Kotarou tells Kazuma that he doesn't like him to which Amane tells him that Hajime is like a member of the family. In his room, Hajime stands there, questioning what a family is, and why he subconsciously saved Amane. His green blood continues to seep from his wound and he hears the alien language in his head. Kazuma and Kotarou continue to speculate who the other rider is. They agree that the other rider must be good because he saved Amane. The Undead from before jumps down in front of them and attacks Kazuma again. He is able to transform again and pulls the monster back. He draws out his Blay Rouzer and fights the Undead before quickly losing it. The Undead continues to attack him, taking complete control. Kazuma remembers Shiori's Words again and fights back claiming that it's his job. Chalice appears again and knocks Blade out of the way before attacking the monster. He easily sends the monster flying into the forest. He uses a Tornado card through his bow and unclips the Undead's buckle by shooting an Arrow at it. He throws his own card and seals it. Blade confronts him asking him if he is an ally before Chalice turns on him. He says that everyone is his enemy and continues to attack Blade. He leaves soon after, and Kazuma cancels the transformation question who that rider was. He answers a phone call from Shiori who tells him that she contacted Chief Karasuma's Hard-drive which has resulted in a holographic image of Karasuma lying asleep on a table.
- Kazuma Kenzaki (剣崎 一真 Kenzaki Kazuma): Takayuki Tsubaki (椿 隆之 Tsubaki Takayuki)
- Hajime Aikawa (相川 始 Aikawa Hajime): Ryōji Morimoto (森本 亮治 Morimoto Ryōji)
- Shiori Hirose (広瀬 栞 Hirose Shiori): Yumi Egawa (江川 有未 Egawa Yumi)
- Kotarō Shirai (白井 虎太郎 Shirai Kotarō): Terunosuke Takezai (竹財 輝之助 Takezai Terunosuke)
- Haruka Kurihara (栗原 遙香 Kurihara Haruka): Kaori Yamaguchi (山口 香緒里 Yamaguchi Kaori)
- Amane Kurihara (栗原 天音 Kurihara Amane): Hikari Kajiwara (梶原 ひかり Kajiwara Hikari)
- Kei Karasuma (烏丸 啓 Karasuma Kei): Kazuhiro Yamaji (山路 和弘 Yamaji Kazuhiro)
- Housewife: Maki Mabuchi (馬渕 真希 Mabuchi Maki)
- Locust Undead (Voice): Katsumi Shiono (塩野 勝美 Shiono Katsumi)
- Plant Undead (Voice): Hiroyuki Fukuyama (福山 弘幸 Fukuyama Hiroyuki)
- to be added
- Cards used:
- Tackle, Slash, Change
- Tornado, Change
- Cards obtained:
- Kick, Bio
- As part of Super Hero Time, this episode aired alongside Bakuryuu Sentai Abaranger episode 49, Ep. 49: Break In! Abare Final Decisive Battle.
- Viewership: 10.0%
- This episode shares its name with episode 6 of Kamen Rider Ryuki and episode 10 of Kamen Rider 555, though in the case of 555, different Wikipedia translators working on the different series' episode lists results in the 555 episode being known as "The Enigmatic Rider" to most English-speakers. "Enigmatic" and "mysterious" are equally valid translations for the word nazo, as in "Nazo no Raidaa" (謎のライダー), the name of all three episodes in Japanese.
- With a rating of 10.0%, this along with the previous episode was the highest rated episode of Blade, 2.1% above the series average.
- A catch-up of the first twelve episodes of Kamen Rider Blade was aired as part of the Super Hero Max 2012 special event on TV Asahi. 
- The opening was updated with footage of the riders riding on their bikes.
Kamen Rider Blade Volume 1 features episodes 1-4: The Indigo Warrior, The Mysterious Rider, Their Secret... and Immortality's Mysteries. It was released on DVD on August 6th 2004, with a total run time of 94 minutes.
Blu-ray Box 1 comes with 16 episodes.