Lt. Krieg (John D’Aquino) encounters what appears to be a giant sea monster while operating a seaCrab in the Tonga Trench. The creature grabs the vehicle and spins him around, and when he finally is released, he sees glowing rocks along the ocean floor.
He tells Ortiz (Marco Sanchez) about his ordeal, and Krieg swears him to secrecy when he shares the rocks. But in order to get a secure vidlink to Murray, a potential buyer, he has to also tell Lucas, who believes he is the only one Krieg told.
Meanwhile, Captain Jack Clayton, portrayed by Yaphet Kotto (Alien, Homicide: Life on the Street) is inspecting seaQuest and administering proficiency tests. He tells Captain Bridger (Roy Scheider) that the crew is too science-heavy, and he tries to prove his athletic superiority: “Great upper body strength, flat gut, and the buttocks of a sixteen year old. You don't believe me, do you? Feel for yourself. You think I'm kidding, I've cracked walnuts with these cheeks.” Bridger ignores him and walks away, and who can blame him?
Greed has taken over the crew as Chief Crocker (Royce D. Applegate) and O’Neill (Ted Raimi) realize that Lucas and Ortiz know about the stones. Chaos erupts as everyone fights over the seaCrabs, so they can go out and find some rocks on their own. Krieg takes command and orders everyone to conduct the search in an organized fashion and adds not to let Bridger know.
Bridger and Clayton have a swimming race to see who is the fastest, and Bridger wins. Clayton isn’t done with the tests, though, and issues a lights out procedures. Lucas finds himself with an armful of the rocks as he tries to find somewhere to hide them, but Clayton and Bridger easily see him in the dark and chase him, eventually finding him as he’s dumping them into the moon pool. Darwin knocks them right back out, and Krieg walks in and reluctantly explains what happened.
Everyone begins to believe Krieg's story when the sea creature returns and wraps its tentacles around the ship, reaches up through the moon pool, knocking Lucas over and freaking everyone out. But Dr. Westphalen (Stephanie Beacham) remembers something Darwin said and interprets that the creature sees them as light. So they turn the lights off and the creature leaves. Dr. Westphalen runs tests on the rocks and realizes that they're essentially fish poop, created from a bioluminescent diet. The organisms are already beginning to decompose, leaving only waste where there was once a glowing wonder.
As punishment for all the upheaval Krieg's discovery caused, Bridger confines him to his quarters with the poop, which has now begun to have an unpleasant aroma. But, happily for everyone else, Clayton realized during the attack that the science team's expertise is needed on the vessel, and the crew's response to the unknown threat is to be commended. He departs, with everyone taking a deep sigh of relief. Everyone, of course, except Krieg.
Bob Ballard explains that while the sea creature in the episode doesn't exist, there is a fish thought to have gone extinct 66 million years ago that was recently rediscovered. Recently being in the 20th century: the coelacanth. Derived from the Greek "hollow spine" coelacanth is "Lazarus taxon, an evolutionary line that seems to have disappeared from the fossil record only to reappear much later."
He also mentions that the light deep-sea creatures produce is called bioluminescence. The Smithsonian has a fascinating gallery of bioluminescent fish: here.
Where are they now?
Jonathan Brandis. Prepare yourself. This doesn't end well. As a young actor, Brandis appeared in The Neverending Story II and Stephen King's IT. His role as Lucas catapulted him into teen magazines, and he remained with seaQuest until its final episode in 1996. Around that time he did some made for tv movies and voiced Mozenrath in the animated series Aladdin on the Disney Channel. But after seaQuest ended he struggled to regain the fame he had found on the show. He is said to have been dismayed at his career, and on November 12, 2003, he hung himself at the age of 27.