Noelani and McGarrett agree a homicide victim was killed on the bottom of the ocean. The victim was Jason Kamaka, a former professor of oceanography. So the murderer had to be one of four people at the underwater lab Neptune One, where Kamaka worked. So McGarrett plans to send a team to investigate the murder. Equipped with stun guns, Junior, Tani, and Adam will go down in the submersible. Adam discovers fresh blood when they arrive at the lab. It's from Jim Walker, who says he was attacked by Marcus Nash. They find Nina Kane and Linda Brady, who is having a panic attack and needs oxygen. Junior and Tani go to search for Nash. They find him in a hidden lab, and Tani knocks him out with a stun gun. They discover the lab isn't for studying lava rocks as Five-0 had been told. It's Yttrium, a rare mineral used in many products that the lab's benefactor, Claude Nostromo, manufactures. Brady and Nash were using the secret lab until Kamaka stumbled on it. Brady said she and Nash were working together all night, so he couldn't have killed Kamaka. All the data on the secret research had been erased. Nash said Kamaka could have all them killed. Nostromo said there's no backup for the research data, so the data had to be erased at the lab. Oxygen levels have been tampered with in the lab. Kane was Nostromo's mole on board. She locks everybody in and runs, taking the submersible and trapping everyone else with oxygen dwindling. Danny and McGarrett figure out the sub has been hijacked. Tani rescues a disoriented Junior after he released a buoy.  Steve dives with oxygen for his passed-out colleagues, then makes it back to the surface. Nina gets caught after she gave herself the bends trying to escape.