Translation:We are pursuing the criminals.
The plural suffix is always optional in Klingon -- tlhIngan can thus be "a Klingon, the Klingon, Klingons, the Klingons". tlhInganpu' can only be plural: "Klingons, the Klingons".
It's never wrong to add the plural suffix, though, even when the verb prefix shows that the subject and/or object must be plural.
So you could equally correctly say HeSwI'pu' DItlha'lI'.
True in Welsh at least for numbers -- which is also where I personally tend to drop the Klingon plural suffix most often.
wej paq = tri llyfr = üç kitap = három könyv = "three book"
wej paqmey "three books" would also work, though, unlike Turkish or Hungarian. (But a bit like Welsh optionally, especially with higher numbers: tri o lyfrau.)
But: "how many books?" can only be paq 'ar?, not *paqmey 'ar?.
The plural is almost always optional and a noun that is not marked with a plural suffix might be read as either singular or plural. Often context will tell you which it would be. If we were already talking about multiple criminals, you would probably assume I meant the plural, but if we had only been talking about one, you might be pretty confident that I meant it as singular.
However, there is another clue within this sentence. Sometimes grammatical context can make it clear. In this sentence, the use of the DI- prefix leaves no room for a singular object. For HeSwI' to have been singular the prefix would have to be wI-. The DI- prefix can only have a plural object. So in this sentence HeSwI' can only mean "criminals" (plural).
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-taH does not preclude an action with a natural conclusion, though.
For example, paqvam vIlaDtaH is fine for "I am reading this book", even though reading a particular book has a natural conclusion (when you've reached the last page).
You can also -- but do not have to -- say paqvam vIlaDlI', which is more specific about making progress to a known goal.