- The Y can stand for 'yesterday' (the idea presented here rather than Yesterday Dopant) or 'Yuko' which can apply to 'Yukie' as well.
Meet Fuwa Yuko, a woman in search of Yesterday... actually the name of her cat, Kinou meaning 'yesterday', but Shotaro has no problem taking her case right away. Because, he reasons, giving the cat a deliberately Meaningful Name means that it's representative of a special memory - or maybe just because she's hot. (The woman, not the cat...) Interestingly, she does mention that someone told her about Narumi Detective Agency specifically, even if it's for a little case like this. As if reacting to the clock striking twelve, Shotaro gets off his duff and on the case...
At the same time, Terui Ryu, with Jinno and Makura in tow, are questioning one of the city's, shall we say, more prominent land developers, in his lavish penthouse suite. It turns out two of his Co-Dragons have lapsed into comas, with odd little hourglass-symbols appearing on their skin, following unusual accidents - apparently one walked right into traffic of his own accord, while another drove at full speed into a busy area - but their suspect's alibi is airtight. Or watertight, as it were, since he'd spent that whole time in his personal indoor pool. So there's not much for Ryu to go on... until the clock strikes three, whereupon the man suddenly springs to his feet, whipping off his clothes and taking a leap just like he probably would with that pool of his - except they're at the penthouse, and he's taking a flying leap off the building. Good thing Ryu's right there to catch him... just before he lapses into a coma, like the others. With a hourglass symbol of his own.
The action moves out of Narumi Detective Agency and all the way to one of the city's less prominent landmarks, Fuuto Hall, where Shotaro and Akiko are helping Yuko find that cat any way they can. They get a little sidetracked by the posters announcing an upcoming event at the hall, organised by Sonozaki Saeko herself - though it's actually the use of the Fuuto-kun mascot that caught Yuko's eye. When even deploying the Memory Gadgets doesn't work, Shotaro busts out what he calls his 'secret weapon'... meowing and prowling like a cat. Which is technically his way of understanding what the client (read: the cat) is going through and using this connection to find them - but he earns a slipper to the head for it anyways. Although Yuko does subscribe to that theory too, meowing on her own as they keep looking, eventually heading in different directions...
Turns out the upcoming event is a speech by Saeko, CEO of the Digal Corporation (basically a front for Museum), a job which can sometimes bring unwanted attention, as Wakana brings news about a threat to her life - which she naturally shrugs off. At the same time, the search for the cat takes an unexpected turn, as Akiko is first to notice a creature covered in cogs and an hourglass on its head, in plain sight on the rooftop. Without wasting any time, Shotaro heads right after it, going into Double to follow it as it takes a flying leap off the roof, then heads straight into the hall. Double manages to corner it as it heads into the stage, but it scores a direct hit with a blob projectile at him - which doesn't seem to do anything. Double pulls the Joker Extreme on it in response... but only succeeds in destroying the podium as the creature heads out a side exit, making its escape. At least the danger has passed, as Akiko and Yuko turn up all right, and Yuko even managed to find the cat. So, all's well that ends well then... apart from Akiko rushing after Yuko for payment...
The next day, Saeko prepares to head out for Fuuto Hall to carry out the event on schedule, shrugging off Wakana's worry for her by stating that making this sort of decision is a must if one is to carry on the Sonozaki family business - while remaining blissfully unaware of Ryuubei's words to Wakana concerning the great Gaia Valve the week before. At the same time, Shotaro can't get Yuko out of his head, as Akiko tries her hand at coffee making and Ryu pops over to seek Phillip's help again. Now he's got three victims instead of two, all from the same land development company, putting their acquisition of a southern area in Fuuto on hold. And things are about to get a lot worse as Yuko is already waiting outside of Fuuto Hall - with a Dopant Memory on hand. And jacks in. Yes, the creature Shotaro spotted the day before was her the whole time...
...but there's not much Shotaro can do at this point. Or much of anything, as the clock strikes three - except spring to his feet, explaining that the cat is representative of a treasured memory while pointing at the files on the three victims, then taking some time over which hat to wear before setting off. Just like he did the day before. Phillip figures it out immediately and does a quick search, arriving at the Yestarday memory, which can cause its victims to replay all their actions 24 hours ago. Which explains the three victims at last - the man who walked into traffic had been walking his dog, and the one driving into residential areas at full speed had been on a long highway. And the most likely cause at this point is the blob that the Yesterday Dopant shot at Double, which in this case was Shotaro' physical body, thus affecting only him. And Akiko is completely helpless to stop him as he chases a nonexistent Dopant up the same building, then takes a flying leap off the roof. As himself. Fortunately Phillip activates the Double Driver from his end, allowing Shotaro as Double to land harmlessly, but having Phillip in there as well doesn't help at all - the houglass mark left by the blob shot at Double is still there, and he goes after nothing in particular with weapon in hand, straight into Fuuto Hall - which is now packed with an audience as Saeko gives her speech at the podium. It's obvious what Yesterday Dopant is after now, as Double directs his Finishing Move at Saeko herself...
Except this time Ryu as Accel is on hand, forcing Double out of the building, where the Xtreme Memory manages to find Double and latch on to form CycloneJokerXtreme - which has the added effect of shaking off the Yesterday marker blob and releasing Shotaro's mind. And this time, the Yesterday Dopant is actually still outside instead of slipping off, whereupon Double busts out the Prism Bicker and starts attacking - until Accel mentions that he's already figured out who it is. And Yesterday reveals itself to be Fuwa Yuko.
Except that's not what he meant.
Fuwa Yuko is just the alias used by Sudou Yukie, younger sister of one Sudou Kirihiko - except she's not here to avenge him, but outdo him, demonstrating her power to become the driving force of Museum itself. And she already has an edge, as simply reverting to Yukie is all it takes to prevent Double from landing his Finishing Move on her. The game is pretty much over, as Yukie just walks off, leaving Shotaro to figure out where he went wrong - although everyone around him agrees that she'd set out to use Double for her own goals, Shotaro believes that something in this picture just doesn't fit... but what?
Meanwhile, Yukie and Saeko (as Taboo Dopant) finally meet up formally, with Yukie explaining that her stunt with Double was basically a demonstration of her power. Which isn't nearly enough to get her Saeko's attention - whereupon Yukie basically words it as a threat. She's even mastered the Stealth Hi/Bye to avoid Saeko's wrath... but it'll take a little more than that, now that Dr Isaka has been secretly listening in...
Yukie makes another stop at the place where she practically grew up, a kindergarten in the southern suburban area of Fuuto - where part of her motive is revealed. Apparently the 'land development people' have been trying to force the kindergarten people off the land for some time, up till recently where they stopped showing up entirely - or maybe lapsed into comas. Turns out this is where Yukie and Kirihiko spent most of their childhood, after which they kept in touch in their own way... up until Kirihiko's final phone call to her, asking how she's doing overseas and mentioning Narumi Detective Agency on his way from the harbor where the Bird Dopant was defeated, to meet up with Saeko. About the only thing wrong in this scene would be Shotaro, who's anticipated her next move and went to the kindergarten ahead of time. All because he'd finally had her figured out - she was someone who couldn't let go of the past, what she called 'yesterday'.
Interestingly, the same could be said of Saeko, who's been holding on to Kirihiko's Nasca Memory this whole time - or at least Wakana assumed it to be. She's obviously got something in mind for it, possibly involving Dr Isaka, but he has other concerns, like Yukie's claims about 'mastering' the Yesterday Memory. Turns out her marker blobs are composed of the very neurotoxin that should be driving her mad from extended Dopant Memory usage, but channeled instead into a unique method of mind-controlling others. Which is the only reason Isaka wants Saeko to meet up with Yukie again.
Meanwhile, Shotaro is still out to talk Yukie round, insisting that the case she hired him for isn't really over. He believes Yukie is more like Kirihiko than she'll admit, telling her about the truth behind the Bird Dopant case - that Kirihiko stuck his neck out for some kids, just as Yukie has by protecting the kindergarten. Unfortunately Yukie is already fed up to hell with him, and goes into Yesterday to bring him to an end, but Shotaro goes into Double just in time and manages to turn the tables - forcing yesterday to release a rain of marker blobs over the city, making Double turn his attention to them, as she flees for it.
Saeko finally decides to meet up with Yukie again and add her to the ranks of Museum, even tempting her with added power from even more Dopant memories, complete with Dr Isaka as living proof. Without further ado, Yukie reaches out to seal the deal with a handshake - before jacking in and forcing a marker onto Saeko. Turns out this was Yukie's plan the whole time, an one way or another, this was going to end.
Shotaro's been hoping to avoid that, but he and his crew have been searching for an entire day and still haven't found Yukie anywhere. They manage to circle right back to the last place where Shotaro saw her, near the kindergarten, and sure enough, she states outright that she's been out for revenge the whole time. Then she brings out Kirihiko's red-spot cravat - actually the second time, from Shotaro's standpoint - and retelling how she found it blowing in the wind when she first arrived. Then she starts screaming for revenge, and that's when our heroes finally get it - Yukie's been hit with the Yesterday affliction herself, with her own marker on her neck. Which is confirmed as they watch her jack in and become Yesterday Dopant, re-enacting the events of the day before, as Dr Isaka as Weather Dopant steps in to provide appropriate exposition...
Yes, Yukie scored a direct hit with her marker blob on Saeko, but Dr Isaka had already improved on the Taboo Memory through his experimentation, allowing Saeko as Taboo to shake off the marker and send it back at Yukie. And re-enacting the same few actions repeatedly over the last 24 hours is about to consume Yukie with brain damage, until those moments are all that is left of her. There is no time to waste, except Rider time, as Shotaro and Phillip go into Double immediately, but Weather prevents them from doing anything to help Yukie's condition. Naturally, Terui Ryu shows up, claims Dr Isaka's life for his own and goes into Accel immediately. While he's not any more effective against Weather than normal, it allows Double to concentrate on Yukie, as Shotaro suggests that they call up Xtreme form and try to replicate the effect it had on themselves back then.
Sure enough, Xtreme Memory shows up of its own accord, and Double goes into CycloneJokerXtreme to pull a Prism Break on Yesterday Dopant - successfully snapping the Yesterday marker in half, as Yukie passes out. There's nothing more that can be done now, so Double turns his attention to Weather Dopant, breaking his Neck Lift on Accel before he can fry Ryu in his own suit, and busting out a new Finishing Move, the Double Xtreme Rider kick. Isaka just slips away... but he'll be feeling that in the morning.
Finally it's over, as Yukie comes to just long enough to admit that seeking out Shotaro and company was the only thing she did right... And suddenly she forgets who they are. Just as predicted, and ironically too, as Yukie is finally free of the past that has tormented her to this state. It's up to regular medical science to undo the damage, and Yukie herself to set things right... but it won't be that easy. Maybe it won't be that far off, either. Possibly. Hopefully.
There's no hope for Akiko's coffee making skills, though.
- Call-Back: Shotaro reuses Isamu Bitou's Dope Slap, and we have 'land developers' that behave more like Yakuza.
- Flaw Exploitation: Shotaro will not harm unarmed humans. Yukie exploits that by changing back before he as Double can strike her down.
- "Groundhog Day" Loop: Inverted. The premise of this story is all about replaying the actions of the same day as time passes normally, to fatal extents.
- Harsher in Hindsight: LunaTrigger displays the ability to shoot down a thousand floating marker blobs before they infect all of Fuuto with the Yesterday affliction... gee, that would have come in really handy for dealing with all the floating spiders in Message for Double in Movie Wars Core...
- Mundane Utility: When Saeko gets the first threatening letter, Dr Isaka burns it... as Weather Dopant.
- Revenge: Double Subverted (pun not intended). Yukie was indeed trying to avenge her brother Kirihiko. Her plan merely involved getting into Museum, and close to Saeko, under the pretense of "surpassing" her brother.
- Shout-Out: Prism Break, anyone?
- Tragic Keepsake: Yukie found Kirihiko's cravat when she got to Futo. The painful part? It was Yukie's gift to Kirihiko, to congratulate him on getting engaged.