|This article is about a/an episode in Kamen Rider Drive.|
What's Go Shijima's Reason to Fight? (詩島剛が戦う理由はなにか Shijima Gō ga Tatakau Riyū wa nani ka) is the twenty-seventh episode of Kamen Rider Drive. It features the in-series debut of the Ride Booster Set and the Booster Tridoron combination following their initial appearance in the Rider Grand Prix of Super Hero Taisen GP: Kamen Rider 3.
Shinnosuke and the rest of the Special Unit are now more confident thanks to the new Ride Booster support vehicles. Kiriko and Go eventually get into an argument whether to trust Chase or not as a Kamen Rider. Later, Shinnosuke joins a joint investigation from the Central Government Office where Brain (in his human form) tries to frame Shinnosuke, whom is already aware of Brain's involvement in the division. Meanwhile, a girl named Yoriko seeks Go's help as her brother was attacked by a Roidmude.
While several citizens playing at the neighbourhood, a strange Roidmude throws a Neo Spider Viral Core.
At the Kuruma Driving License Centre, Rinna introduced the Ride Booster Set that would be useful for all three Kamen Riders. Go however finds it irritating to count Chase as their ally, earning a slap from Kiriko. As Go left them with Kiriko in remorse, Genpachiro called them that the First Division wanted the cooperation with Special Investigation Unit. Yoriko Soma tried to called for the Unit's help but find Go instead and call for his help that his little brother had been kidnapped by a Roidmude. During the meeting, it was reported that 37 citizens of Yumemigayama Village had been injured from serious attack and Mitsuhide Nira decided to hand over the case to Shinnosuke, being the son of the late officer Eisuke. They were even introduced to Noumi, whom obviously to Kiriko and Shinnosuke is none other than Brain in disguise.
Yoriko told Go of her brother's background as a real estate agent of a town while he secretly relate it to his relationship with Kiriko. Go decided to take the case on his own and promise to save the woman's little brother. At outside, Shinnosuke as Drive tried to force Brain Roidmude to reveal all of the secrets of 001. Though Brain only reveal several small details, however, he quickly take this as an opening to frame Drive. While Brain express his success to 001/Makage, an officer entered his office and demanded answer of a recent monster attack but 001 altered his memory to thought this as an accident. Shinnosuke was suspended from being Drive and the Tridoron has been towed away. While Shinnosuke decided to quit as an officer, Jun hold him off, telling that his father would died in vain. Go and the woman realised that recent cases of household areas being attacked are related to his brother's disappearances.
The Unit founded it's real estate agent, Ryo Soma, and both Gen and Rinna pose themselves as new neighbours with Shift Colorful Commercial covers Shinnosuke's disappearances while Shinnosuke went undercover quietly. Shinnosuke even bumped into the similarly undercover Go and Yoriko, while noticing the woman's relation to Ryo and Kiriko visited Chase, having required his help and him as well wanted the Shift Speed Prototype to be fixed in hopes of recovering his lost memory. Suddenly, some of the residents turned wild, with Genpachiro as well being hold off by Shinnosuke and realised a logo on his hand, as well as those affected. The true culprit appeared, Seeker Roidmude, whose true motif is to amplify humanity's negative emotions to find the one worthy of fusing with him. After Seeker knocked the affected victims for deeming the unworthy, an angry Go transformed into Mach.
While fighting Seeker, two other Roidmudes showed up and assist him in fighting Mach. As he about to be finished, the Ride Booster Set appeared and saved him. Mr. Belt ordered Shinnosuke to transform but hesitated after remembering Jun's words. Chase appeared and allow himself to take over for Shinnosuke, transforming into Chaser and and defeated the two Low Classes with Shingou Ax. However, Mach furiously attacked Chase, where Seeker revealed that it was all a part of the plan and leaves. As an untransformed Chase is about to be killed, Shinnosuke stepped in and stopped Mach. As Chase leaves and the injured citizens being carried by several paramedics, Go tearfully revealed that his motive for destroying Roidmudes is because they were his father's creation, and many people died in the Global Freeze because of his mistake. He desired to destroy them to atone for what his father had done.
- 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)
- Jun Honganji (本願寺 純 Honganji Jun): Tsurutaro Kataoka (片岡 鶴太郎 Kataoka Tsurutarō)
- Rinna Sawagami (沢神 りんな Sawagami Rinna): Rei Yoshii (吉井 怜 Yoshii Rei)
- 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)
- Roidmude 001 (ロイミュード００１ Roimyūdo Zero Zero Ichi)/"Soichi Makage" (真影 壮一 Makage Sōichi): Masami Horiuchi (堀内 正美 Horiuchi Masami)
- Roidmude 050 (ロイミュード０５０ Roimyūdo Zero Go Zero, Voice): Kiyohito Yoshikai (吉開 清人 Yoshikai Kiyohito)
- Eisuke Tomari (泊 英介 Tomari Eisuke): Tareyanagi (たれやなぎ)
- Kiriko Shijima (teen): Yui Kato (加藤 結 Katō Yui)
- Go Shijima (Child): Ryu Hashizume (橋爪 龍 Hashizume Ryū)
- Yoriko Soma (相馬 頼子 Sōma Yoriko): Haneyuri (はねゆり)
- Ryo Soma (相馬 良 Sōma Ryō): Sota Goto (後藤 崇太 Gotō Sōta)
- Police: Keigo Ayukawa (鮎河 圭吾 Ayukawa Keigo), Masaru Ōbayashi (大林 勝 Ōbayashi Masaru)
- Boy: Raiki Komino (小美野 来希 Komino Raiki)
- Father: Ryota Ashidachi (足立 りょう太 Ashidachi Ryōta)
- Mother: Ai Tanaka (田中 あい Tanaka Ai)
- Senior Commissioner Matsui (松井): Junichi Kubozono (窪園 純一 Kubozono Junichi)
Shift Cars & Signal Bikes
- Car Used:
- Type Change: Shift Speed (once attempted), Shift Formula
- Tire Exchange: N/A
- Type Used:
- Type Speed (once attempted), Type Formula
- Bike Used:
- Form Change: Mach
- Signal Exchange: N/A
- Form Used:
- Form Change: Chaser
- Form Used:
- When Chase inserts the Signal Chaser Signal Bike into his Mach Driver Honoh, instead of making a proper Signal Bike inserting sound, it made the sound of a Shift Car being inserted into the Shift Brace.
- When Roidmude 082 is explode, his Core not appeared.
- As part of Super Hero Time, this episode aired alongside Shuriken Sentai Ninninger episode 9, Shinobi 9: Ninjutsu vs. Magic, the Great Battle!.
- Viewership: 5.5%
- Closing Screen Shift Car/Signal Bike
- Roidmude Count
- Answer to the episode's question: To atone for his father's sins.
- The opening is updated to add Medic as one of the Roidmude generals and Chase as Kamen Rider Chaser.
- However, during the scene where the Shift Cars are flying through the show's main characters, Chase is still seen together with Heart and Brain.
- Rei Yoshii's (Rinna Sawagami) 33rd birthday, falling on March 18th 2015, was celebrated during filming of this episode. 
Kamen Rider Drive Volume 7 features episode 25-28: Why Has a New Battle Started?, Where is Chaser Going?, What's Go Shijima's Reason to Fight? and Why Were the Families Targeted? 
Collection 3 comes with 12 episodes.