This episode begins with a child finding a severed hand while swimming in the ocean. McGarrett is informed of the case while he is eating lunch with Joe and trying to get a feel for whether or not Joe is trying to help him get to the bottom of the surveillance video featuring his father and Wo fat. Joe tells him to be patient but it appears Steve thinks that the process is taking suspiciously long.
Back at headquarters Steve and Danno meet with Max to discuss their victim. The man (Blake Spencer) was shot and then cut up by a boat propeller. When they go to his home to investigate they find out he is a salvage diver and treasure hunter. They also find some old coins in his house and when they take them to an expert they establish that they may in fact be from a historical shipwreck called the Princessa Del Mar. This leads them to what they believe is a rival salvage diver (Jesse Billings) who has made no bones about the fact that he is looking for the same ship and is taking on investors to help pay for his work. After viewing a promo video aimed at luring investors in they see that the deceased worked for Jesse Billings.
Meanwhile… Steve goes to Catherine to see if she can check with the DOD and find what is taking so long to process the video Joe gave them. Lori visits Blake’s mother to inform her of her son’s death but thinks better of it when she finds that the woman is suffering from alzheimer’s and has regressed to thinking Blake is still a child. Chino Ho finds an old wallet in the deceased’s dive bag. The lab thinks the wallet was submerged for over 50 years. This means that Blake was not salvaging the Princessa Del mar as they suspected.
When they find out that the Princessa Del Mar coins are fakes they figure out that Billings had faked their discovery to keep his investors on the hook. Blake must have discovered this deception but Billings insists that he did not know Blake was on to him and thus had no reason to kill him. The money they found in the old wallet is from the World War II era and it leads them in a new direction though. They call out the coast guard to get help locating Bake’s boat, which they assume must still be out there. They find it anchored about 40 miles off the shore of one of the islands. The boat proves to be the scene of the crime. They find blood and an expelled slug which tells them someone else was on the boat and that this person betrayed Blake.
Steve and Joe dive down to find out what Blake was salvaging and they find a WWII ambulance with an old skeleton in it. Since they believe the body is likely a military man they decide to look through old records to see if they can identify the man. Steve finds a record of a downed officer’s body being transported via boat when the ambulance carrying his coffin was lost at sea. Lori recognizes the man from a photo she saw at Blake’s mother’s home. It is her father and Blake’s grandfather. The body was lost and they were never able to properly bury him which was agonizing for the family. This dive was very important to Blake but it seems unlikely anyone would kill him over this discovery. But then Joe is able to get an ID on the body found in the front of the ambulance. The man was dishonorably discharged from the military after stealing 2 million dollars of overprint currency. He must have hid the money in the casket of Blake’s grandfather to get it off the base and THAT discovery would be motive for murder.
The team finds that the equipment rental owner that Blake used had boats in the vicinity of Blake’s dive on the day he died. When they go to his office they find the old casket that had been pulled from the ambulance and the rental owner dead inside it. Lori is able to lead them to the killer when a ring imprint is spotted on the man’s skull. She interviewed the owner of the ring, a rival diver, earlier that day. When they try to apprehend him on his boat a shoot out ensues but they manage to take him alive and arrest him. The diver killed Blake and the rental owner to get the money. Case closed.
In other news… Steve says goodbye to Catherine because she is being transferred across the world but she does tell him that the DOD never received any tape from Joe. Danny asks a new lady out on a date and Kono is covering her tracks after hacking Chin’s account. At a military funeral held for the lost officer, Steve asks Joe why he trying to keep him from the truth about his father. Joe tells him that he is not the only one afraid to find out where this rabbit hole goes. Joe isn’t keen on potentially smearing the McGarrett name just to get at the truth, which won’t bring Steve’s dad back. The episode closes with Steve disregarding this and saying he has to know.