|This article is about a/an episode in Kamen Rider Drive.|
Why Did the Important Memories Disappear? (大切な記憶はどうして消されたのか Taisetsu na Kiyoku wa doushite Kesareta no ka) is the thirty-first episode of Kamen Rider Drive. This episode marks the debut of Roidmude 001's evolved form, the Freeze Roidmude.
A young man named Toru Fujiki has asked the Special Investigation Unit for help. Toru says that his father, who runs a factory in town, has suddenly disappeared. To make matters worse, all memories of Toru's father have been erased from his family and the factory employees.
Shinnosuke met Brain, where he revealed that he now know who 001 is. As Brain in his Roidmude form fights Shinnosuke, the Shift Cars and Mr. Belt appear to his aid but even as Drive, Go/Mach interferes and attacks Drive again, in the end Drive fells into the sea as Mach and Brain walk away.
At the Unit's office, Mr. Belt expressed his anger at Shinnosuke for putting his life in a gamble. As he leaves, Genpachiro enters and reveals their recent client, Toru Fujuki. His family runs a small factory and he always gets into a fight with his father. One day, he feels guilt of what he had done before when witnessing his father suffering from old age working. He wanted to apologize by taking over his work. However, his dad doesn't seem to know him and when Toru returns with his dad's favorite food, he vanished. He asked his mother and his workers where his father had gone but all of them replied that he had been gone for 10 years. The Unit suspects 001 behind this case.
At the factory, Shinnosuke and Kiriko senses Heavy Acceleration particles. Toru confirms that he did sensed it's presence while out to buy some dorayaki. Shinnosuke wonders what 001 wanted to do with Toru's father. 001 met Medic and asked her for some of her Reaper Legion soldiers. 001 commences that it's time for them to achieve evolution by using human emotions. He also noted that all of the executives had their corresponding emotion: Brain has envy, Medic has adoration, Heart has joy and 001, still unspecified but if these emotions had been spiked up, they will gain their final evolution. 001 later changed into his Advanced Form, Freeze Roidmude. Freeze states that Shinnosuke would become the catalyst to achieve his final evolution and Heart states that he had already found his desired person. Chase witnessed his memory and theorizes that if they achieve Over Evolved, it might as well related to the Promised Number.
Kiriko visited Mr. Belt, where he was still recovering from the fear of losing Shinnosuke, having dreamt of Protodrive's defeat every night. While Shinnosuke spends the late night searching for clues, he received an e-mail from Mister X. The next day, Shinnosuke had finally get the bottom of the case, using Mister X's mail which reveals the medical record which from 2005 where at that year, the whole country had been alarmed of a huge epidermic. The National Defense Bureau released the the information of the virus and mandated the test. The one in charge of that was Soichi Makage and in the end, no one was ever infected, meaning that a large agenda was hidden in there.
Genpachiro phoned Shinnosuke, where he revealed that Toru had forgotten about them. Shinnosuke witnessed a snowflake scar behind his ear, meaning that Toru had also fallen victim to 001. Meeting Nira, Shinnosuke asked to arrange a meeting with Soichi Makage as he even revealed the truth about Makage but Nira refuses to believe even ripping off the evidence. As Tomari goes to the Tokyo Central Forum and meets Makage, exposing his true identity and fights with two Reaper Legion soldiers. Chase appears and attacks the Reapers while Shinnosuke confronts 001, asking what is he after but 001 shoots an energy snowflake that render him unconscious, with Kiriko and Mr. Belt arrived too late. 001 later freezes the whole place, wiping out the public's memory of him as a Roidmude.
While Chaser fights the Reapers, Mach comes to aid them though he was later outmatched again and the Reapers were killed. Shinnosuke awakens, having retained his memory and approaches Makage again despite Mr. Belt's protest. As Shinnosuke and the others approach 001 again, the latter reveals his evolved form and how his ability works: it freezes the human memory and shatters it from inside. But other than Shinnosuke, he also had met one person whom also proved immune from the effect: Eisuke Tomari. Long ago, Eisuke suspected the police activity somehow being manipulated from inside. He suspected Makage and unmasked his true identity but even when 001 tried to erase it, it had no effect on Eisuke. Fearing the latter, it had awakened his corresponding emotion: humiliation. Shinnosuke as Drive Type Formula tried to attack him but proven futile due to his enormous ice power, even the Pit Crews being incapacitated.
- Shinnosuke Tomari (泊 進ノ介 Tomari Shinnosuke): Ryoma Takeuchi (竹内 涼真 Takeuchi Ryōma)
- Kiriko Shijima (詩島 霧子 Shijima Kiriko): Rio Uchida (内田 理央 Uchida Rio)
- Chase (チェイス Cheisu): Taiko Katono (上遠野 太洸 Katōno Taikō)
- Go Shijima (詩島 剛 Shijima Gō): Yuu Inaba (稲葉 友 Inaba Yū)
- Kyu Saijo (西城 究 Saijō Kyū): Kenta Hamano (浜野 謙太 Hamano Kenta)
- Genpachiro Otta (追田 現八郎 Otta Genpachirō): Taira Imata (井俣 太良 Imata Taira)
- Mitsuhide Nira (仁良 光秀 Nira Mitsuhide): Kisuke Iida (飯田 基祐 Iida Kisuke)
- Narration (ナレーション Narēshon), Drive Driver's voice (ドライブドライバーの声 Doraibu Doraibā no koe): Chris Peppler (クリス・ペプラー Kurisu Pepurā)
- Brain (ブレン Buren): Shota Matsushima (松島 庄汰 Matsushima Shōta)
- Heart (ハート Hāto): Tomoya Warabino (蕨野 友也 Warabino Tomoya)
- Medic (メディック Medikku): Fumika Baba (馬場ふみか Baba Fumika)
- Freeze (フリーズ Furīzu)/"Soichi Makage" (真影 壮一 Makage Sōichi): Masami Horiuchi (堀内 正美 Horiuchi Masami)
- Toru Fujiki (藤木 徹 Fujiki Tōru): Daisuke Sawamura (澤村 大輔 Sawamura Daisuke)
- Iwao Fujiki (藤木 巌 Fujiki Iwao): Jin Hirao (平尾 仁 Hirao Jin)
- Haruko Fujiki (藤木 春子 Fujiki Haruko): Tomoko Abe (阿部 朋子 Abe Tomoko)
- Makage's Assistant/Roidmude 028: Kiyoshi Ishii (石井 淳 Ishii Kiyoshi)
- Special Police/Roidmude 092: Kenji Tominaga (富永 研司 Tominaga Kenji)
- Eisuke Tomari (泊 英介 Tomari Eisuke): Tareyanagi (たれやなぎ)
Shift Cars & Signal Bikes
- Car Used:
- Type Change: Shift Speed, Shift Formula
- Tire Exchange: N/A
- Type Used:
- Type Speed, Type Formula
- Bike Used:
- Form Change: Signal Mach
- Signal Exchange: N/A
- Form Used:
- Form Change: Signal Chaser
- Form Used:
- to be added
- As part of Super Hero Time, this episode aired alongside Shuriken Sentai Ninninger episode 13, Shinobi 13: Burning! Ninja Sports Day.
- Viewership: 5.5%
- Closing Screen Shift Car/Signal Bike
- Roidmude Count
- Answer to the episode's question: Because the people fell victim to the Freeze Roidmude's mind alteration abilities.
- This is the first appearance of Reaper Legion Roidmudes since episode 22 as well as the first time they fight the original Reaper who has now become Kamen Rider Chaser. 028 and 092 are also the first of the Reaper Legion whose numbers do not end with the so-called number of death, "four" (四 Sì).
- However, 28 and 92 are numbers that are divisible by four, into 7 and 23 respectively.
- This is the 750th episode of Kamen Rider in the Heisei era (that is, since the start of Kamen Rider Kuuga).
- The flashback scene in episode 10 where Protodrive defeated was added in this episode, where Mr. Belt separated with 001 picking him up and Protozero can be fallen.
- This is the first episode since Go's first appearance where he has no lines at all, despite his full appearance.
Kamen Rider Drive Volume 8 features episode 29-32: What Really Happened to the Robbery Event?, Who Reveals the Real Criminal?, Why Did the Important Memories Disappear? and What is Waiting At the End of Evolution? 
Collection 3 comes with 12 episodes.