En esa novela, la trama es sencilla, aunque el argumento cambia en el último capítulo.

En esa novela means in that novel.

The preposition en is needed because the sentence is talking about what happens within the novel, not identifying the novel as the subject of the sentence.

• En esa novela, la trama es sencilla = In that novel, the plot is simple
• Esa novela es sencilla would mean That novel is simple, which sounds less natural if you specifically want to talk about the content inside it.

So en sets the context.

These are demonstratives, and they show distance from the speaker.

• esta = this near me
• esa = that near you or already mentioned / not especially close
• aquella = that over there farther away

In many everyday cases, esa is the normal choice for that.

So:

• esa novela = that novel
• esta novela = this novel
• aquella novela = that novel over there / that more distant novel

A learner should also notice agreement:

• novela is feminine singular
• so the demonstrative must also be feminine singular: esa

This is one of the most interesting parts of the sentence, because in English both can often be translated as plot.

A useful distinction is:

• la trama = the overall plot structure, the thread of events
• el argumento = the storyline, the content of what happens, sometimes the central line of the story

In real usage, the two words can overlap a lot, and native speakers do not always keep a strict distinction. But in a sentence like this, the contrast suggests something like:

• la trama es sencilla = the plot structure is straightforward
• el argumento cambia = the storyline or direction of events changes

So the sentence is playing with two closely related literary terms.

Spanish often uses ser for essential or defining characteristics, and estar for states or conditions.

Here, sencilla describes a general characteristic of the plot:

• la trama es sencilla = the plot is simple

That is treated as a quality of the plot, so ser is the natural verb.

Using estar sencilla would sound odd in this context, because it would suggest a temporary state, and sencilla is not normally used that way here.

Because it agrees with la trama, which is feminine singular.

In Spanish, adjectives usually agree in gender and number with the noun they describe.

• la trama = feminine singular
• so: sencilla

Compare:

• el argumento es sencillo
• la trama es sencilla
• los argumentos son sencillos
• las tramas son sencillas

This kind of agreement is very important in Spanish.

Yes. Here aunque means although / even though.

It introduces a contrast:

• la trama es sencilla = one idea
• aunque el argumento cambia en el último capítulo = contrasting idea

So the structure is:

• statement + aunque
  • contrasting statement

Examples:

• Es caro, aunque bueno = It is expensive, although good
• Estudia mucho, aunque está cansado = He studies a lot, although he is tired

In your sentence, aunque connects two facts that seem slightly surprising together: the plot is simple, yet the storyline changes at the end.

Spanish often uses the present tense to describe the content of books, films, and stories. English can do this too.

This is sometimes called the historical present or narrative present.

So:

• el argumento cambia en el último capítulo = the storyline changes in the final chapter

Even though the events of the novel are fictional or already written, the present tense is natural when summarising them.

You could also see this in sentences like:

• En la película, el protagonista muere al final
• En el cuento, la niña encuentra una llave

In el argumento cambia, el argumento is the subject of the verb cambia.

That means:

• el argumento = the storyline / plotline
• cambia = changes

So the sentence means the storyline changes.

You would only need an object pronoun like lo if something else were changing the storyline:

• El autor lo cambia = The author changes it

But that is not what this sentence says. Here, the storyline itself is presented as changing.

Because en expresses location within the story: in the final chapter.

• en el último capítulo = in the last chapter

By contrast, a often expresses direction or movement toward something.

So:

• cambia en el último capítulo = it changes in the final chapter
• va al último capítulo = he/she goes to the final chapter

Also note the contraction:

• en + el = usually stays en el
• a + el = becomes al

That is why you see en el here, not al.

Usually, yes, but the exact sense depends on context.

In el último capítulo, it clearly means the final chapter.

Common meanings of último include:

• last / final in a sequence
• sometimes latest / most recent, depending on context

Examples:

• el último capítulo = the final chapter
• la última vez = the last time
• las últimas noticias = the latest news

Here, because a novel has an ordered sequence of chapters, último naturally means final.

It is very natural and helpful here.

En esa novela is an introductory phrase that sets the context, so a comma is commonly used:

• En esa novela, la trama es sencilla...

Without the comma, the sentence would still probably be understood, but the punctuation with the comma is clearer and more standard in this kind of written sentence.

It helps separate:

• the context: En esa novela
• from the main statement: la trama es sencilla

Yes. All of those can work, depending on tone.

• simple = the most direct translation
• straightforward = often sounds more natural when talking about plot
• uncomplicated = also possible, a bit more descriptive

So la trama es sencilla could mean:

• the plot is simple
• the plot is straightforward
• the plot is uncomplicated

The best choice depends on the style of translation, but grammatically sencilla is a very common adjective meaning simple.