I'm not entirely sure as it could be a very specific or fan - made story. But generally in the Ben 10 Ultimate Alien series, Ben Tennyson has a new Omnitrix which gives him access to ultimate forms of his aliens. The big story might involve him facing new villains, protecting the Earth and the galaxy, and dealing with the consequences of using his powers.
I'm not entirely sure about the exact plot details of 'ben 10 ultimate alien the big story'. It might involve Ben Tennyson using his various alien forms to face some big threats and challenges, but that's just a general guess.
Ben Tennyson is definitely one of the main characters. He's the one with the Omnitrix and can transform into various aliens. Gwen, his cousin, who also has magical powers, might be another main character. And of course, there's Grandpa Max who has been guiding Ben in his adventures.
Ben 10 Ultimate Alien is about Ben Tennyson who has the Omnitrix which allows him to transform into various alien forms. In this series, he has new challenges and enemies. He unlocks ultimate forms of his aliens, which are more powerful. He continues to protect the Earth and the universe from different threats.
In this story, the central character is Ben Tennyson. His ability to transform into ultimate aliens using the Omnitrix makes him the key figure. Gwen, his cousin, is an important part of the story too. Her magical abilities often come in handy during their adventures. Grandpa Max, with his wisdom and combat skills, is another mainstay. Additionally, there could be antagonists or supporting characters that interact with Ben, Gwen, and Grandpa Max. These could be other aliens, whether they are friends or foes, that add to the complexity of the story. For example, some of Ben's alien forms like Heatblast or Four Arms might have a more prominent role in certain parts of the story, depending on the situation and the challenges they face.
Ben Tennyson is the central character. He has the ability to turn into various ultimate aliens, which is a big part of the story. Gwen Tennyson, his cousin, might also be a main character as she has her own powers and often helps Ben. And then there could be Kevin Levin, who has powers related to absorbing matter. These three characters often work together to face the threats in the 'Ben 10: Ultimate Alien' universe.
One idea could be Ben going back in time to prevent an alien invasion that in the original timeline led to the destruction of his hometown. He could use his ultimate alien forms in creative ways to outwit the invaders before they even start their attack.
Definitely Ben himself. His ability to transform into different aliens makes for interesting scenarios in different time periods. For example, in the past, he could transform into an alien that was thought to be a myth, causing all kinds of chaos or amazement.