I'll start this mini-review by stating that this was a huge let down from the first two episodes of the new series. Although I liked some of the themes it presented, it just felt like such a filler episode.
This episode picks up right where episode 2 left off. With Thundera destroyed and King Claudus dead, Lion-O, Cheetara and Tigra set out for the Sea of Sands to exact revenge on Mumm-Ra. As they depart, they meet up with Wily Kit and Wily Kat, and Lion-O reluctantly lets them join in on their travels. Once they reach the ocean, they encounter a group of fish-like pirates, who are straight out of an H.P. Lovecraft novella and are looking to kill a creature called the Ramlak - who drank all the water in their ocean (don't ask if you haven't seen it... it's stupid). The Thundercats agree to help them find and kill the creature, while simultaneously the crew wants to eat the Thunderkittens (again, don't ask). As you would have guessed, Lion-O saves the day and kills the Ramlak, releasing all of the water back into the sea (I have no idea how the creature was large enough to hold that much water... I shouldn't have asked).
The best part of the episode is the very end, so I won't ruin it for those of you who still haven't seen it and are curious about it. Although it was cool to see Lion-O cut shit up with his sword, I don't think I can recommend this episode.