Adrien was seeing if he could participate in a video game contest after taking a picture of the qualifying rounds with his trusted camera (seeing if gaming could be what he'll do with his life) with the other contestant Max when he won, meaning that he will compete in the contest. Then Marinette came up to see if she can participate (if she won that would mean she'll take Max's spot) and won. Max seemed calm so Adrien wasn't worried, too bad he was wrong and Max was Akumatized
Adrien was waiting to practice gaming with Marinette when he saw on the news that The Gamer (Max's Akumatized form, dubbed that way by the news) was attacking Paris citizens. So, he left with the excuse that he was getting footage for Alya for her blog. After leaving, he transformed. After arriving on the scene as Cat Noire, she immediately texted Ladybug "To get here!" on Cat Noire's staff. Luckily Marinette while looking for Adrien heard from her mom about the Akuma and that's why he left. So, she (while hiding) transformed into Ladybug and left to join Cat Noire who was fighting the Akuma's mech suit (controlled by said Akuma) with her staff while the mech suit attacked with shoots which Cat Noire deflected with her staff while getting closer to it. That's when Ladybug tied it up with her yo-yo, allowing cat noire to Cataclysm it which she did. Thus, allowing them to easily take down Gamer who was powerless without his mech suit via Ladybug tieing him up with her yo-yo and Cat Noire snapping in half his Akuma object. Then Cat Noire said "Cataclysm!" Again, activating her special cataclysm (you know the one that's green and gives her a new look) and with it frees all the people and cars that the Gamer has shot.
Marinette and Adrien finally get to practice for the contest where Adrien discovers that he isn't good at co-op mode even with Marinette's bracelet, this convinces him to drop out of the contest and let Max take his place. Which he does.