The premise of the story, which is based on the first Justice League story arc in the New 52, is that Darkseid and the hordes of Apokolips have attacked the Earth, bringing the Justice League together for the first time.
After the credits, there is a a shot of the ocean, strewn with dead fish and other sea life. A craft rises out of the water, and with it Aquaman's brother Ocean Master. He's carrying a dead, old man. Ocean Master says, "The surface dwellers have killed our king. This is an act of war, and they will pay."
Back in August, Geoff Johns (then the writer of Aquaman) teased an Aquaman animated movie coming up. No further details were available from Johns, and Warner Home Video PR rep Gary Miereanu hasn’t commented at all yet–he being the one who generally gets to make such announcements.