Hawkmoth has a new plan to get what he wants, and this time, it doesn't involve Miraculous.

Hawkmoth stopped his attacks, allowing Chat Noir and Ladybug to resume their normal lives. Five years later, Adrien, Marinette, Alya, and Nino are invited to witness a demonstration of a time machine built by Dr. Kubel, the experiment goes awry, causing the disappearance of Kubel, Mayor Bourgeois, and Gabriel Agreste. A week later, as numerous scientists have been reported missing, Alya receives a letter purporting to be from Chat Noir ten years in the future, leading the four to a quaint clock shop in the Paris back alleys. Inside, the old couple show the four another time machine in the guise of a giant clock, and they find themselves in a drastically-changed quasi-steam punk Paris ten years from their present, where they meet the Future Chat Noir. He explains that in this future, Hawkmoth has become the head of a mafia-like mob known as the Akumas and taken control of Paris, and decides to assist him. After briefly returning to the past to look up some files pertaining to an accident which claimed the life of Adrien's mother, Zara Agreste, five years ago, the four return to Paris.

Arriving at the tower, the group learn that the Future Hawkmoth is Dr. Kubel. He reveals to have taken Mayor Bourgeois hostage, revealing that he, along with Zara whom he also had feelings for, were working on a time machine five years ago, but Mayor Bourgeois's desire for the time machine to be a publicity campaign for his election led to the accident that claimed Zara's life. This led to Dr. Kubel's dedication to getting the black cat and ladybug miraculous so he can save Claire's life. He attempts to trap the five, but this is foiled by Gabriel Agreste who had escaped. Gabriel infiltrated Kubel's base of operations to free the other scientists, where Gabriel encounters Zara's near-identical sister, Tara, who assists him. As everyone gathers at the Thames Arms restaurant, where Kubel is also waiting for them, Alya manages to deduce that they are not actually in the future, but in a pseudo-replica of Paris built underground. Adrien also deduces that Kubel is but a pawn compared to the true mastermind, who is revealed to be the Future Chat Noir, whose true identity is Felix, a young man who sought revenge against Kubel and Bourgeois and Paris in general as the explosion that killed Zara also took his parents away from him.

Kidnapping Marinette, Felix escapes to his moving fortress, the true purpose behind kidnapping the scientists, which he brings to the surface to attack Paris. The group head to the surface, where Adrien reveals he is Chat Noir. Adrien takes everyone to Master Fu, who gives Alya and Nino the Fox and Turtle Miraculous, while Gabriel dons the peacock miraculous. With help from Rena Rouge, Carapace, Le Paon, Chat Noir manages to board the fortress and free Marinette, later joined by Tara. After deducing that they need to reverse the flow of power from the generator, they discover Bourgeois is attached to the generator, wired to a bomb. With thanks to Tara's old watch, they manage to reroute the bomb, free Bourgeois and reverse the power flow of the fortress, causing it to collapse. After Chat and Le Paon manage to get everyone to the ground, where Rena Rouge and Carapace manages to evacuate the underground citizens, Chat Noir returns to help Ladybug (revealed to be Marinette) rescue Felix from the fortress before it falls down into the fake future Paris and explodes.

As everyone safely regroups, Felix is put under arrest but promises to atone for his crimes, thanking Gabriel for comforting him as he did the day his parents were killed. It is then revealed that Tara is actually Zara herself, who was teleported five years into the future during the accident. However, her presence had become unstable and would soon send her back in time to the moment of her death, leading Kubel to try to find a way to save her but to no avail. Accepting her fate, Zara has a tearful final reunion with Gabriel and Adrien before bidding farewell disappearing back to her time. Gabriel, although crushed to lose Zara again, finally comes to terms with his loss.

I read somewhere there were fan theories for Kubel being Hawkmoth.

Whose Who:

Professor Layton - Adrien, Alya

Luke - Marinette, Nino

Flora - Marinette

Clive - Felix

Dimitri - Hawkmoth/Kubel

Bill - Bourgeois

Claire/Celeste - Mrs. Agreste