ENG-BD Plummet The total sales of our company have plummeted this month. We need to step things up. Your sentence is a passive form of this equivalent sentence: "(Something) is plummeting the sales of our company this month". So, "sales" is the object of the participle "plummeting". In other words, one thing is plummeting another thing. However, you are not allowed to say that because the verb "to plummet" is always intransitive. That means it cannot take a direct object. It is not possible for one thing to "plummet" another.

ENG-BD Plummet Total sales of our company are plummeting this month. We need to step things up.