The origins of the Reapers, the ancient race of giant robots that returns to harvest sentient life in our galaxy every 50,000 years in the “Mass Effect” series, were a total mystery in the first game.

The second game offered some hints, the ending sequence of “Mass Effect 3” provided some real answers, and the free “Extended Cut” of the ending, released in August after a deluge of online complaints from disappointed fans, provided even more.

Now, “Leviathan,” a $10 chunk of downloadable content, fills in the rest of the gaps and provides missions on several new worlds, along with a couple weapons that were once only pre-order bonuses, and some new weapon mods.

The basic plot: There’s evidence of something out there capable of killing a Reaper outright, and anything with that sort of power is something Commander Shepard needs to enlist. But you’ll have to find it first — and the Reapers are hunting it as well.

The new stuff is interesting, but the way it’s presented is also a little problematic. Because the events of the ending preclude the possibility of continuing to play afterward, as you could in “Mass Effect 2,” you must complete “Leviathan” before heading into the final leg of the game (if you’ve already finished the game, there will be an automatic save file from just before the point of no return.)

If you’ve already beaten the game and seen the “Extended Cut” material, you know the gist of the DLC’s main revelation already, if not the specifics. If you haven’t, “Leviathan” will give a few things away that were originally revealed at the very end of “Mass Effect 3.”