Miraculous: Tales of Ladybug & Cat Noir has a habit of exploring different timelines, especially with the introductions of the time-manipulating Rabbit and Snake Miraculous. Season 3, Episode 19 "Timetagger" teased several epic events to look forward to in the canon Miraculous timeline; however, Season 3, Episode 22, "Cat Blanc" shows a more disastrous future, one that must be avoided should the show's central duo ever hope to become romantically involved.
In this particular episode, Adrien and Marinette finally confess their feelings and become a couple, but their budding romance is not met with comity for long. Instead, their love is tied directly to the destruction of the world in the form of an akumatized Cat Noir -- Cat Blanc. While it's the show's greatest hook, this plainly creates a major red flag for Ladybug and Cat Noir's future relationship.
The events of "Cat Blanc" kick off when Marinette transforms into Ladybug to deliver a gift to Adrien, consequentially exposing her secret identity. Soon after, the holder of the Rabbit Miraculous, Bunnyx, arrives from the future claiming something happens today that causes a huge forthcoming disaster. Bunnyx then uses her superpower, Burrow, to whisk Ladybug to the future where she is met with an apocalyptic-like setting: the Eiffel Tower collapsed on its side, much of the city submerged underwater and the moon, now visible in the daylight, split into two pieces. This is where Ladybug meets Cat Blanc, an akumatized Cat Noir, alone and heartbroken.
While Ladybug fights off Cat Blanc, Bunnyx uses Burrow again to examine how this calamitous future comes to be. Apparently, after Adrien discovers that Marinette is Ladybug, he confesses his feelings without admitting that he is Cat Noir. Both are happy to be in a relationship and their friends are ecstatic, but Gabriel Agreste, Adrien's father (and archvillain Hawk Moth) does not approve. He convinces Marinette that their relationship will only harm Adrien as well as the Agreste brand. So, a distraught Marinette breaks up with Adrien with little explanation.
However, this is all part of Hawk Moth's plan, who then attempts to akumatize Marinette. But Adrien runs out to save her, transforming into Cat Noir in a public setting to do so. Adrien's identity is revealed and Marinette accepts him immediately because he is still Adrien. Unfortunately, Nathalie Sancoeur, Gabriel's assistant and holder of the Peacock Miraculous, witnesses his transformation and informs Hawk Moth that his son is Cat Noir.
Bunnyx skips ahead to a point in time when Ladybug and Cat Noir seemingly manage to corner Hawk Moth, but little do they know, he successfully leads them to his repository beneath the Agreste mansion. Here, he reveals that Adrien's mother, Emilie, is secretly alive, but is merely trapped in a comatose state. Hawk Moth admits his true identity and the real reason as to why he is after Ladybug and Cat Noir's Miraculous: to revive Emilie.
Hawk Moth attempts to convince Cat Noir to willingly hand over his Miraculous to save his mother, but Ladybug reminds him of the consequences that will come if he does so. Cat Noir is placed under immense emotional turmoil over having to choose between the two women he deeply loves, and Hawk Moth seizes the opportunity to akumatize him, turning him into Cat Blanc who attains an infinite power of destruction. His unbalanced mindset turns Ladybug and Hawk Moth to stone then unleashes a calamity he cannot control. Only Cat Blanc remains, akumatized and alone in a broken world.
Though Ladybug is able to reset these events and return things back to normal, this episode reveals some obvious barriers that both characters will need to overcome before they can enter a relationship in Miraculous. Firstly, Adrien only confesses to Marinette knowing that she is Ladybug -- and he does so instantly. By doing so, he does not learn to love both sides of her, only one. Similarly, when Marinette discovers that Adrien and Cat Noir are the same person, she doesn't question anything and gives in simply because he is the Adrien she's loved all along. While Adrien views Marinette solely as Ladybug, Marinette views Cat Noir solely as Adrien and both subconsciously refuse to acknowledge half of each other. For their relationship to work, they'll need to fall in love with both sides of one another.
Secondly, Gabriel interferes both as Adrien's father and as Hawk Moth. The secrets he keeps from Adrien are severe and can be detrimental if revealed under the wrong circumstances. Thus, Hawk Moth will need to be permanently defeated before Ladybug and Cat Noir can confess their feelings. This implies that they cannot truly be together until the end of the series when everything is resolved.
With these barriers in place, Ladybug and Cat Noir's romance seems doomed to fail, but "Cat Blanc" proves that under the right circumstances, their love can thrive rather than cause mass destruction. But before that, Adrien needs to learn to love Marinette while Ladybug needs to learn to love Cat Noir. Until then, Hawk Moth will continue to loom over them with a much bigger plan in mind, one that must be thwarted before he does more harm than good to his own son.