Тот, кто любит хороший детектив, обычно внимательно следит за сюжетом.

Тот, кто is a very common Russian pattern meaning the one who, whoever, or anyone who, depending on context.

• тот = that one / the one
• кто = who

Together, they create a structure like:

• Тот, кто любит хороший детектив... = The one who likes a good detective story...

Russian often uses this two-part structure where English might just say whoever or anyone who.

Because кто is the normal word in the pattern тот, кто... when talking about a person in a general way.

Compare:

• тот, кто любит детективы = the one who likes detective stories
• человек, который любит детективы = the person who likes detective stories

So:

• after тот → кто
• after a specific noun like человек → usually который

Both can translate as who in English, but they are used in different Russian structures.

Because the grammatical subject inside the clause is кто, and кто normally takes singular verb agreement in this kind of sentence.

So Russian says:

• кто любит = who likes

not

• кто любят

Even though the meaning can be general, like anyone who likes, the verb is still singular here.

Because любить takes the accusative case, and детектив here is a masculine inanimate noun.

For masculine inanimate nouns, the accusative looks exactly like the nominative:

• nominative: хороший детектив
• accusative: хороший детектив

So even though it is the object of любит, the form does not change.

A useful comparison:

• любить хороший детектив = to like a good detective story
• любить хорошего актёра = to like a good actor

In the second example, актёр is animate, so the accusative changes.

Because кто любит хороший детектив is a subordinate clause inserted into the main sentence.

The structure is:

• Тот ... обычно внимательно следит за сюжетом
• and inside it: кто любит хороший детектив

Russian sets off this kind of clause with commas:

• Тот, кто любит хороший детектив, обычно внимательно следит за сюжетом.

So the commas show where the relative clause begins and ends.

Сюжетом is instrumental singular.

That is because the verb expression следить за requires за + instrumental when it means to follow, to keep track of, or to watch closely.

So:

• сюжет → nominative
• за сюжетом → instrumental after за

This is something you usually learn as a set phrase:

• следить за сюжетом
• следить за новостями
• следить за ребёнком

It means to follow the plot carefully, to keep track of the plot, or to pay close attention to how the story develops.

This is not the same as simply looking at something.
Следить suggests active attention over time.

So in this sentence, the person is not just reading or watching passively—they are mentally tracking what is happening in the story.

Because внимательно is an adverb, and it describes how the person follows the plot.

• внимательный = attentive, careful — adjective
• внимательно = attentively, carefully — adverb

Here it modifies the verb следит:

• внимательно следит = follows attentively / pays close attention

If you used внимательный, it would need to describe a noun, not a verb.

Обычно means usually, and it can move around more freely than in English.

This sentence uses a very natural, neutral order:

• Тот, кто любит хороший детектив, обычно внимательно следит за сюжетом.

But Russian could also say:

• Обычно тот, кто любит хороший детектив, внимательно следит за сюжетом.
• Тот, кто любит хороший детектив, внимательно следит за сюжетом обычно. — possible, but less natural here

Russian word order is flexible, but the original version sounds the most standard and balanced.

Yes, it can be omitted in some contexts:

• Кто любит хороший детектив, обычно внимательно следит за сюжетом.

This is understandable and natural, especially in more general statements.
But тот, кто makes the structure more explicit and slightly more formal or neatly organized.

So:

• Тот, кто... = more clearly the one who / anyone who
• Кто... = more compressed, a bit more general

Yes, you can make the main idea plural:

• Те, кто любит хороший детектив, обычно внимательно следят за сюжетом.

A useful detail:

• те is plural, so the main-clause verb is plural: следят
• but inside the relative clause, Russian often still uses singular любит after кто

So Те, кто любит... is very common and standard.

You may also hear Те, кто любят... in speech, but learners are usually taught that кто normally goes with singular agreement in that clause.