Learn Swedish: Filmen handlar om två karaktärer som älskar varandra. - The film is about two characters who love each other.

While vem is used to mean "who" in a question (like "Who is that?"), som is a connecting word (a relative pronoun) that translates to "who", "which", or "that". You use som to refer back to a noun you just mentioned—in this case, the karaktärer (characters). Here, som acts directly as the subject of the phrase that follows it.

I thought handlar meant "to shop". Why is it translated as "is about" here?

On its own, handlar usually does mean "to shop" (or "to act"). However, when you pair it with the preposition om to make handlar om, it becomes a fixed phrase meaning "is about" or "deals with". You will use this phrase constantly when describing the plot of a movie, book, or story!

If the characters didn't love each other, where would the word inte (not) go?

It would go immediately before the verb! The sentence would end: ...som inte älskar varandra. In Swedish, the clause starting with som is a subclause (a dependent clause), and a major rule for subclauses is that words like inte must be placed before the verb, not after it.

Is varandra a plural word for "others"?