Miraculous and The Flash crossover: Collision of Worlds. (Book 4)

In Reverse Flash's lair.
Future Lila works on the replica Particle Scanner.

As she works, she thought back to when she first arrived in the past. And how she came to work alongside Reverse Flash.

A flashback shows Future Lila in her lair, in the catacombs. Working on her original time machine, welding the last component into place. She lifts her welding mask, and gazes at the circular gateway she had just completed.

Future Lila: Finally, it's complete. Now all I have to do is set the coordinates, for the right moment in time.

She types away at the keyboard on the device.
Suddenly a loud thud was heard, outside her lair.

(THUMP!)

Future Lila: What the heck was that?!

(Pewpewpewpew! BOOOOM!)

Then there was the sound of laser fire, and an explosion.
Then she heard someone shouting, at the same time, the large steel door to her lair was beginning to rust.

Future Lila: Crud! They're coming!

She finishes setting the proper time coordinates.
At least, she thought she did. At that moment, the door is busted down, and the adult versions of Ladybug and Cat Noir enter the lair.

Future Ladybug: Give it up, Lila! Whatever you're planning, it ends here! Now, give us back your Miraculous!

Future Lila: I don't know how you found my lair. But it'll be a cold day in hell, before I surrender to you. Marinette!

Ladybug becomes surprised, but quickly changes to determination.

Future Ladybug: It doesn't matter if you know who I am. We're still going to take you down!

Future Lila: I'd like to see you try, you parasite! Nooroo, DARK WINGS, RISE!

Future Lila quickly transforms into Falena Falco, and goes on the attack. The protectors of Paris hold their own, as they block the swipes of Falco’s sword stick and knock her back.

Future Cat Noir: You're going to pay for what you did to my father, Lila!

Falena Falco still tries to attack Ladybug. Ladybug whirls her yoyo, using it as a shield. And while Falena Falco is distracted, Cat Noir attacks from behind. Falena Falco whips around, dispatches Cat Noir, and stabs Ladybug in the shoulder, incapacitating her.

Future Ladybug: AAGH!

Future Cat Noir: Ladybug!

He rushes over to check on his injured partner.

Falena Falco: My name is Falena Falco, you wretched alley cat! And your father brought it on himself! I simply took advantage of his imprisonment.

She presses a red button, and the machine activates.

Falena Falco: And where I'm going, you won't be able to stop me!
I'm going to change my future for the better, by getting rid of you two!

She jumps through the portal. And in a sheen of bright light, the machine shuts down. After shielding their eyes, Cat Noir looks at his indisposed teammate.

Future Cat Noir: What do we do now, Marinette?

Future Ladybug: (Winces) We need to find Bunnyx. If she's time-traveled to our past, then this situation just got much more dangerous.

A portal opens up to the past, and Falena Falco lands directly outside the Musée du Louvre, with a thud.

Falena Falco: Oof!

She gets up and dusts herself off, observing her surroundings.

Falena Falco: Wait! This is the wrong spot!
Ugh, I must've punched in the wrong coordinates!
Now how am I-

(SHOOOM!)

She was cut off by a red portal opening, just down the street.

Falena Falco: What the heck?!

(ZOOOOM!)

Suddenly, a rush of red lightning streaks past her.

Falena Falco: Whoa!

She sees a yellow vibrating figure stopping in front of the Seine, on the Pont Des Arts bridge.

After the figure stops vibrating, Reverse Flash looks around, wearing his tachyon enhancer. He takes off his mask.

Thawne: I know you're here, Barry Allen.
AND I WILL FIND YOU!

(ZOOOOM!)

He rushes off, leaving a very confused Falena Falco.

Falena Falco: Who's “Barry Allen?”

As the flashback ends, Thawne comes up from behind, and observes her working on the replica Particle Scanner.

Thawne: How's the device coming, Ms. Rossi?

Future Lila: I'm working on it. It'll be done soon, but I was thinking back on how the two of us met.

Thawne: Were you, now?

Future Lila: Indeed. All because I miscalculated where I was supposed to go.

She turns around to face her yellow-clad accomplice.

Future Lila: Which reminds me, how exactly did you get to this Earth, anyway? And why didn't you age backwards, like Barry did?

Thawne: Well, that's an interesting story.

Multiple flashbacks show Reverse Flash fighting against Flash, Flash gets catapulted into the Speed Force, and Thawne uses the tachyon enhancer to tap into the Negative Speed Force.

Thawne: “After Flash arrived to halt my experiment involving tachyonic particles, I had him right where I wanted him. I was about to harness Flash's speed to enhance my own, when he suddenly disappeared.”

Another flashback shows Thawne arriving on Earth 5102, stealing technology from different tech firms in Paris and beyond, including Tsurugi Industries. He monitors Francois Dupont high school, keeping an eye on who he believed to be Barry, watching the argument he had with Ms. Mendeleev. Once he saw the opportunity, capturing Ms. Mendeleev, and keeping her tied in the boiler room.

Thawne: “Once I modified the Tachyon Enhancer to travel through time and space, I traced his vibrational frequency to this Earth, and monitored him closely.”

Thawne: "And when I found out that Francois Dupont received a new student, and this particular student recently got into a tiff with one of the teachers. I tied her up, and kept her in the boiler room, where nobody would find her. And assumed the guise of a substitute professor, for her class. All as a means of gaining Intel."

As Thawne concludes his interpretation of the events. Future Lila hangs off every sentence.

Future Lila: Huh, that's very interesting.

Thawne: Indeed. I also remember how I met you, shortly afterwards. And how you told me about your desire for vengeance against that polka-dotted pest, and her flea-bitten comrade.

Future Lila: Exactly.

She turns back to the workbench, finishes the adjustments to the device, and activates it.

Future Lila: There. It's complete. Now, all we need to do is find the object we need, to power the device.

Thawne: I don't think that'll be a problem. And what's more, we know exactly who has it.

He presses a button on the keyboard, and the image of the aftermath of Flash, Alix, and her father, Alim Kubdel, pops up on screen.

Future Lila: Of course, Alix Kubdel. Why doesn't that surprise me?