Justice League
The Enemy Below, Pt. 1
S1 E6:
When Superman convinces Aquaman, the king of Atlantis, to come to the surface world to negotiate a peace treaty on behalf of his people, Aquaman's evil half-brother, Orm, takes advantage of his king's absence to seize control of Atlantis. With the help of the Justice League, Aquaman must reclaim his throne before Orm can launch an all-out war against humanity. Part 1 of 2.