Most Definitely Not Accompanying Us

The Hero and his Five-Man Band are gearing up for the Final Battle, heading for the airport for the latest trip to a Temple of Doom, or taking the ransom money to meet the kidnappers. The Tagalong Kid, Annoying Younger Sibling, or the Kid Appeal Character prepares to go likewise. The Hero tells him or her in no uncertain terms that there is no way they are going to be allowed to go along.

Tempting Fate! Anybody unequivocally ordered to stay behind never does. Lock them in their room? They escape with a Bedsheet Ladder. Leave someone to supervise them? Five seconds later, they're a Bound and Gagged Badly-Battered Babysitter. Keep every tarp, trunk, suitcase, or any area of empty space constantly visible to your eye 24/7, or it will be used as a hiding place—often with the intention of the stowaways showing themselves once there's simply no way they can be safely sent back.

This can also be said to The Hero by well-intentioned but misguided mentors trying to keep him or her Locked Out of the Loop. It might also be done to someone not in on the Masquerade, making the motivation not so much their safety as protecting secrets.

The Trope Namer is Basil is Disney's The Great Mouse Detective to Olivia, which accomplishes the trope with a Gilligan Cut. Compare Just a Kid and Stay in the Kitchen, where someone is ordered to stay out of the conflict because of her gender rather than age.

Film - Animated

 * Violet and Dash stow away on Helen's plane in The Incredibles—they both claim it was the other's idea.
 * Carl accidentally takes Russel along with him in Up and realizes there's no way to get him home (although he contemplates using the Bedsheet Ladder to lower him to the ground, he concluses that isn't gonna work).
 * In Cloudy with a Chance of Meatballs, after Flint makes the plan to stop the Meateroid, Brent asks what he can do. Flint makes him "President of the Back Seat," then heads out with Sam, but Brent is quick to follow.

Literature
"Queen Lucy: But where is his goosecap highness? King Edmund: Not in the front line, and that's good enough for now."
 * Corin in The Horse and His Boy sneaks into the battle he was told to stay out of. He beats up the dwarf assigned to keep an eye on him, disguises himself, and keeps out of sight until the battle starts, when he reasons everyone will be too busy to notice him.


 * In The Last Battle, King Tirian tries to send Eustace and Jill home after they release him and Jewel, before he and his allies go into battle against the Calormenes. Jill refuses, but she feels like Eustace ruins the drama of the conflict by pointing out that they have no way of getting back to their world even if they wanted to.
 * Go ahead and just try to tell Harry Potter and his Power Trio to stay out of things and forget about the Sorceror's Stone, the Chamber of Secrets, Sirius Black, and the Order of the Phoenix. Just try...

Western Animation
"Webby: Please, no lectures about how dangerous this is. I'm coming with you, and that's final. Scrooge: Aye, I insist you come with me, Webbigail. I could'na possibly trust you to stay here. Webby: Huh? Why not? Scrooge: Because keeping an eye on poor Huey, Dewey, and Louie is the most important job in the world. Now, who could I trust to do that? Webby: Oh, I can do it! Scrooge: Oh, I don't think so. Webby: Oh, yes! Don't worry, Uncle Scrooge! You can count on me! Scrooge: I'm sure I can."
 * Although Scrooge McDuck was only too eager to take his nephews along on his adventures in the comics, the triplets were sometimes the victims of this trope in DuckTales, Webby even moreso:


 * Gosalyn repeatedly on Darkwing Duck.