What Is This Thumping Heart? (このときめきって、なに？, Kono tokimeki tte, nani?) is the fourth episode of the SSSS.Dynazenon series.
Monster of the week: Didoras (ディドラス, Didorasu)
Yomogi catches a cold and goes to sleep. Gauma gets Yume to visit him. Yomogi is confused by Yume's sudden visit, but the relationship between the two slightly unravels. When Yume hears that a monster had appeared, she leaves Yomogi with his cold and heads for battle.
The episode begins with another of Yomogi's training session with Dynasoldier under Gauma's tutelage where he must focus to maintain its stance at the same time. Despite his efforts, there's something or two that distracted him, resulting the robot to fall in the process. As the young man got tripped and fall into the river while exiting Dynasoldier, he notices Yume's absence and inquires Gauma about it. Gauma informs Yomogi that she has an appointment with someone, surprising the young man as the first thing that came up in his mind being the girl's unhealthy habit of breaking promises. As such, he wonders whether he should phone her to see whether she continues such habit.
Meanwhile, Yume engages in a conversation with Kanae (the alumni of chorus club Kano used to be part of) where she inquires the latter about her impression regarding her late sister at a coffee joint. Through her, Yume learns that Kano was no-nonsense when it comes about practice and other things yet cheerful at the same time where the latter trait of hers made her likeable, much to her pleasant surprise as she never saw such side of her late sister. Kanae then suggests Yume to ask Urita for the video of their chorus practice and shares her his phone numbers. Before they part ways, Kanae asks Yume whether she intends to continue ask around about her late sister, in which Yume states that she is out of curiosity over who Kano truly was.
While working at Inamoto's store, Yomogi's mood doesn't improve since he still distracted for some reasons. Noticing this, Inamoto surmises that he has a crush on someone (Yume) and encourages him to ask her out on a date after confirming it by teasing the young man by having him carrying more boxes. Just then, Yomogi begins to sneeze, surprising Inamoto who assumes that he apparently unwell. Meanwhile at Chise and Koyomi's residence, Koyomi is surprised to see Inamoto sent him a message to ask him out.
Mirroring Yomogi's situation, Yume finds herself teased by Mei at the karaoke room they visit in the same day because the latter becomes curious by her friend's unexpected change.
Later that night, somewhere in the city, another kaiju begins its move, using its power to turn random things into 2D renditions of themselves.
In the next morning, Yomogi's cold becomes apparent as he, his mother, and Gauma are having a breakfast together. As Mrs. Asanaka expresses her mild surprise to learn Gauma works day and night (though Gauma brushes it being pale compared to his previous life 5000 years back), the latter takes a note of his friend's condition thus allows him to skip his training for the day. Despite arriving late due to the traffic, Yomogi nevertheless manages to join his classmates at the school's auditorium during which they are introduced to four exchange students where one of them happened to be the member of Kaiju Eugenicists, much to his and Yume's surprise.
As of Sizumu's arrival into their school as one of the exchange students was shocking enough, the shortest member of Kaiju Eugenicists winds up become Yomogi and Yume's classmates and even chooses to sit behind the latter (which Yomogi finds irritating for some reasons). The two are understandably baffled by the unexpected turn of events, though Sizumu luckily opted not to confront them for the sake of his cover. Instead, as with Juuga, Sizumu shares some intriguing info to Yomogi and Yume; kaijus he and his comrades control are not their creations, but rather grown from people's wills and exist to free them from something they bound upon – all they do is to use them to create a world where such kaijus are needed. This didn't stop Yomogi and Yume from informing Gauma about the development in their school, however, and as expected, their friend shows up to challenge Shizumu for approaching the two. In the ensuing ruckus, Gauma unceremoniously kicked out for his troubles.
Unknown to everyone in the school, the rest of the Kaiju Eugenicists are watching from the coffee joint across the street. The annoyed Onija wonders why their friend should trouble himself with his current plan, in which Juuga reassures him that Sizumu is working on a plan since he is able to hear kaiju's voices.
We then shown a CCTV footage of a parking lot where one of the cars, which had darkest coloration compared to the rest, suddenly turns into a small kaiju.
In the next day, Yomogi's cold worsened that he effectively had to stay at home. Meanwhile, the worried Yume asks Gauma whether he got into trouble for his recklessness on the other day, and Gauma states that he's fired from the part-time job he worked on at the time, thus he had to be careful next time. Gauma then informs Yume that Yomogi is indeed too sick to join the training and it apparently has something to do with her letting him waiting in the middle of the rain few days back. As such, he asks her to visit him after school. Koyomi on the other hand, asks for a day off in the next week, surprising Chise as he technically has every day off.
Visiting Yomogi, Yume informs her friend the reason of her absence a day prior; she indeed met someone who knows Kano (Kanae), but that person doesn't know her late sister that much thus directed her to someone else (a guy named Urita). During such conversation, it becomes apparent their feelings for each other begin to unfold, but it is soon interrupted by Gauma who alarmed them about the next kaiju attack. Since Yomogi is not in good shape for fighting, Yume inquires him Dynasoldier and states that they'll have someone else to pilot it in his stead just for the imminent battle, but not before telling him that she left him some snacks.
Meanwhile, Kaiju Eugenicists have found their next kaiju, a red and blue crocodilian one with a pair of yellow drill/cannon like protrusions on its back and silver plates on the sides of the neck and limbs. As unimpressed as he is with its dimensional shift power, Onija lets Mujina control it and set it loose upon the city in its giant form. Gauma and co. soon arrive to engage the monstrosity with their respective Dynazenon component and him lead the charge by releasing a barrage of Dynadiver's Burst Missiles. To his frustration, the kaiju counters the incoming projectiles by converting them into 2D holographic rendition of themselves causing burst missiles to harmlessly phase through it. Things become complicated when Yume shows up with Dynasoldier's replacement pilot ready, and the pilot in question turns out to be none other than Chise. To Koyomi and Gauma's further shock, Chise successfully activates Dynasoldier on ease only to lose control over it because of lacking its intended owner's experience, prompting Koyomi to save her life when the kaiju attempts to crush her beneath it.
Back to Asanaka residence, Yomogi mulls over what Sizumu told both him and Yume before contacting Yume to hear his friends' progress. Upon hearing that they suffer great disadvantages because of Chise's inexperience, Yomogi makes up his mind to join them despite his condition, but forgets the fact that Yume can quickly pick him up to their position with Dynawing until she shows up with it. Switching with Chise after Yume combines Dynawing with Dynasoldier to assume its Wing Combine configuration, Yomogi wastes no time to have Dynasoldier deliver a crippling kick to disorient the crocodilian kaiju just long enough for everyone to properly form Dynazenon. With the group now able to properly fight the beast, Gauma and co. begins to turn the tide of the fight against the crocodilian kaiju, culminating to them reconfigure Dynazenon to Dynarex and subsequently finish it off with Blazing Inferno Rex Roar. Hilariously, as with the previous battle, Onija had a close call where one of the kaiju's remans nearly struck him as his comrades calmly makes their leave, much to his chagrin. With the kaiju gone, everything it converted with its power return to normal.
In the aftermath of the battle, Chise profusely expresses her gratitude to Yomogi and promises to train as his backup pilot better while Gauma wonders whether Dynazenon can be piloted by anyone beside them to begin with. As they are on their way back home, Yomogi asks Yume whether he can join her in her research about Kano since he wants to know about her (Yume) too before being interrupted by the latter's sneezing.
In the next day, Yomogi attends the class since he already healed and pleasantly surprised by Yume's absence. As it turns out, she caught his friend's cold on the other day, much to her chagrin...
- 4.4: It's a Bit Scary When You Talk Quickly (早口だとなんか怖い, Hayakuchi dato nanka kowai)
- Yomogi visits Yume, who caught his cold. While chatting, they somehow end up talking about strange breathing techniques that Yume and her mother practice, and that Dynawing was just in plain sight in Yume's room.