The first trailer for Part 2, "Determination," includes a very brief preview of the third installment, to be entitled "Confession." This preview is completely silent, and shows Patamon flying away from behind, then a screen saying "Goodbye, Takeru." The next shot is Takeru's face from the side, but only from the nose down. Considering the trauma Takeru went through in the first season, this probably isn't something good.
Even worse: The context in which Patamon says "goodbye" is meant to denote a permanent farewell. Wherever Patamon is about to go, be it his grave or just away, it seems that this time he's not coming back...
Initially it appears Ken has become the Digimon Kaiser again and he's destroyed the Primary Village and possibly killed yet another Leomon. It wasaverted in the next movie, which reveals it's not really Ken but a dark version of Gennai. Of course, the latter part begs the question of just what happened to the real Ken.
Leomon's death in part 2. Okay, we probably all saw it coming now that it's become a Running Gag, but consider this: It's indicated that this is the same Leomon from season one, as they said all the Digimon would be reborn in Primary Village. But this time he dies in the real world. Remember in 02 how Wizardmon was stuck as a ghost? If a Digimon dies in the real world, they don't get reformatted as an egg. In other words, Leomon's not coming back this time...
Made even more jarring with the scene preceding Leomon's death and his Last Words. Meicoomon's back but scarred by the experience so Leomon held Meicoomon high up like a parent trying to comfort their child, only for Meicoomon to stab him in the chest, before utterly slashing him into pieces. Yikes.
Leomon: What's wrong...? It's all fine...now.
Plus, Leomon ALMOST made it the entire movie without dying.
Also from the second one, the arc that Mimi and Joe are going through. Mimi starts to show a more selfish and egotistical side, to the point that even Koushiro is disgusted with her (though Sora defends her and tells Koushiro that it was out of place), and the realization hits her hard. Joe, meanwhile, starts to feel like a coward for missing out on his friends' adventures. The discussion between the two of them as they discuss this is heartbreaking. Fortunately this gets resolved in the end.
Joe hates himself, believing himself to be a coward for not helping his friends, and its only made worse when he ends up alienating Gomamon with the belief that he thinks he's a hindrance to Joe and believes that keeping him away means Joe won't have to fight. These two fighting with each other is downright heartbreaking.
There is also the talk between Mimi and Meiko about selflessness, where Mimi explains why she acts the way she does:
Meiko: "Mimi, youre amazing."
Mimi: "Huh? How come?"
Meiko: "A 1st year volunteering, doing and saying as you please. I cant do any of that."
Meiko takes her digimon's disappearance hard, falling into depression and blaming herself. Made worse when she realizes that Patamon is infected.
Takeru realizes that Patamon is infected and ends up being bitten. He does his best to cover it up and try to isolate Patamon without alarming them, but Patamon quietly stares at him, showing that he realized all along. When they eventually come to terms with it, they break down crying.
Takeru being hit the hardest by this makes sense when you remember that he's the only one of the eight whose Digimon already died before, as Angemon against Devimon, something that haunted Takeru throughout 02. Especially when, during the moment itself, he certainly had no idea that Angemon would just revert to an egg. Add to the fact that, when they all later learn of the reboot, while he knows that Patamon will be reborn again, unlike last time, it will assuredly not be the same Patamon he knew.
Made no better by one of the 02 Drama CD, which states that it took TK years to write about what happened to Angemon. Lord only knows how hard this would've been on him.
Patamon even ends up telling the other Digimon that he hopes that they'll put him down if he turns completely.]]
As it turns out ALL of the Chosen Digimon were infected. After hearing that a Reboot may be the only way to save the world they all begin to spend time doing things with their partners all while hiding the truth about the infection. Even worse when Meikuumon finally returns to the Real World the Chosen Children all come together to restrain her and each become fully infected and turn on each other until only Metal Greymon and Kabuterimon are left. Metal Greymon is the last to become fully infected and immediately evolves into War Greymon leading the charge against the sole Herakle Kabuterimon who desperately tries to force them all back into the distortion just as the reboot occurs.
And when he manages to snap them out of it to allow him to push them into the portal, they all give one last group hug before entering the distortion.
Though Tentomon had the option to back up his data, he refuses to do so if it means everyone else is working hard on stopping Meicoomon. Before he evolves, he tells Koshiro goodbye.
The Reboot effectively wipes the Digimon of all their memories including everything about their partners. They even slowly retreat when the Chosen Children try to approach them after meeting again.
Part 4: Loss
According to the synopsis, even after evolving, only Piyomon is cautious of Sora.
Adding to the above, simply watching Yokomon coldly reject Sora's attempts to reconnect is heartbreaking.
Which is a sad ironic echo to the beginning of Digimon Adventure when Sora was the only one to reject her digimon's undying love and affection.
In the trailer, we can briefly see Meicoomon, looking sad, scared and lonely in the Digital World. Poor thing...
Watching the Digidestined attempt to reconnect with their partners is a little sad when they all used to be best friends. Tanemon freaks out when she thinks Mimi is going to eat her, and Tsunomon, who had always been a bit shy but also the only one who ever understood Yamato, awkwardness and all, hides when Yamato comes near. Yamato even has to pretend he doesn't have anything to say so that he'd feel more comfortable around him.
After the brief attempts at bonding when everyone's coming back together to discuss what they should do next, it's pretty evident that Mimi was crying off-screen.
Whereas the other Digidestined are upset that the Digimon don't remember them anymore, Hikari doesn't think the reboot is completely bad. Note that as she states so, it's interposed over a shot of Salamon/Plotmon snuggling in her arms, who now no longer has the memory of being forced to grow up fast under years of abuse from Myotismon/Vamdemon.
Even so, while it wasn't verbally mentioned, the loss of her memories means she has completely forgotten about Wizardmon and his Heroic Sacrifice, as well as turning her into a completely different 'Mon with the erasure of her personality. Likewise as it was before, their relationship is the most different (and uniquely heartbreaking) than the others in that they have to literally start a whole new relationship from scratch.
Taichi briefly wonders what they mean to the digimon anymore when their attempts to digivolve didn't work.
Gennai strangles Meiko to near-death just to anger Meicoomon. Seeing her go limp is heartbreaking. The only saving grace is that posters for the fifth movie reveal her alongside Kari, meaning that she'll survive this ordeal.
The reveal that Maki was doing this to see her partner again. This only gets sadder when Maki finally find her partner revived, only for it to not remember her and be scared of her.
Part 5: Coexistence
Just when you thought Hikari's remark about Tailmon not having any memories of her abuse from Vamdemon, or the death of Wizardmon was a nice heartwarming moment, the poster for Part 5 features Ophanimon Falldown Mode. How much worse can it get for her?
Turns out what's worse is Tai falling off a cliff.
Seeing Koromon devastated and asking for Tai is heartbreaking to watch.
That Tai fell because he first pushed Meiko and Yamato out of the way, and then told Omnimon to protect them instead of himself.
Maki's apparent death: drowning in the waves of the Dark Ocean while crying out for Tapirmon in total anguish.
The ending theme "Aikotoba" is a complete swerve from the high-energy rock and pop songs that usually take that place in the franchise, closer to an elegaic piece of vocal chamber music.
Part 6: Our Future
Daigo pulling a Heroic Sacrifice to get Tai and the 02 cast back to the Digital World. The worst part is that he was already gravely wounded, and probably wouldn't have made it back safely. Before the lab explodes, he encourages Tai before collapsing. Tai's reactions to this are also pretty tear-jerking.
Just the fact that Gatomon and Meicoomon are stuck in a situation that says "And I Must Scream" is this, as they're Forced to Watch as Ordinemon carries out Yggdrasil's will. Gatomon outright says that Meicoomon is the one who's suffering the most.
Just when it looks like everything will be okay, with Gatomon being saved and Meicoomon remembering Meiko, Yggdrasil takes command of Ordinemon and keeps her in her destructive form, forcing the DigiDestined to have to put her down.
Meiko and Meicoomon saying goodbye.
The ending theme: butter-fly, performed by the entire cast. It starts out with Kouji Wada's vocals, but without the backing music, as though his spirit is singing from beyond the grave. Then the music kicks in, and the entire cast starts singing. Then, at the very last verse, Wada's vocals join the whole cast one final time to close out the song. This is the original Digimon franchise's equivalent to the final ending theme of Jo Jos Bizarre Adventure Diamond Is Unbreakable. If they wanted to pay the ultimate tribute to an artist so heavily involved in the series' legacy, they fucking nailed it. Rest in peace, Kouji Wada.
Compounding this is the fact that it isn't just Kouji Wada and the cast, it's also them plus Ayumi Miyazaki (singer of "Brave Heart") and Ai Maeda (singer of ending themes "I Wish" and "Keep On" & Japanese voice of Mimi) which adds to the feeling of the song honoring Kouji Wada's legacy. It's easy to get emotional just listening to it, considering both of them have been involved with the franchise in a musical capacity from the beginning, all the way through Digimon Frontier.