If Tom’s appearance results in a drain on Huck's ingenuity, what does Tom’s arrival represent in The Adventures of Huckleberry Finn?
Huck's ingenuity is no match for Tom Sawyer's insistence on a preference for high adventure. Though Huck had shown an amazing ability to lie spontaneously and spin tales to explain himself when caught in a jam, he finally finds himself in a serious situation when Jim is captured.
Up to this point, Huck had used his ingenuity to save himself and had not had to save Jim (for the most part). When his lies fail him in certain situations, he remains safe because he is not in a definitively illegal state like Jim, who is a runaway slave.
However, when Jim is capture, Huck must summon the moral courage and the right ideas to rescue Jim from captivity. He does not fail in this effort, but is supplanted upon the arrival of Tom Sawyer.
In the last third of the book, Huck defers to Tom Sawyer, whose outlandish schemes to free Jim direct the action. Huck is no longer in charge, and his moral quest appears to have been abandoned.
As a light-hearted and episodic adventure, it is fitting that a new dynamic be introduced at this stage in the novel. A new source of humor is introduced as Huck's more mature and practical imagination is contrasted to Tom's wild, fanciful and fantastic imagination.
Not only does Huck defer to Tom Sawyer's ingenuity, the novel defers to it as well as Tom determines the action of the remainder of the book. This change helps to bring the book back to a light note at the moment when it threatens to become dark (as Jim is returned to bondage).
While Huck and Jim, as characters, would like to focus on the serious issue of freeing Jim, Tom Sawyer insists on making the situation fun, displaying a Don Quixote-style adherence to fantasy as it is derived from adventure novels.
Neither Huck nor Jim approve of Tom's "adventures," although they feel compelled to submit to his authority in such matters.
The appearance of Tom Sawyer, diminishing Huck's need for ingenuity, guides the novel to a comical and happy close and serves to help avoid a more serious-minded conclusion.