As kids play in the surf a hand floats up from the ocean. Max and Five-O get involved to find out whose hand it was and where the rest of him is. After Danny suggested he was shark food, Max explained that the cut was man made not shark

The hand turned out to belong to a salvage diver named Blake Spencer, when his body turned up it had a bullet wound in it as well as missing the hand. Five-O began searching for where he was diving. Their search first took them to Jesse Billings (Blake’s boss) who was salting dive sites with fake coins to get investments. 

Lori then found that Blake had rented a boat and from checking with the boat owner it was still out. The Coast Guard found the boat anchored. Joe and Steve dove down to see what got Blake killed. When they got to the bottom of the ocean they found an old ambulance lost in 1942. 

The ambulance turned out to have the remains of Blake’s grandfather which he tried to recover to help his sick mother have some closure on her father’s death. It seems that 2 million dollars was also smuggled out in the casket and that is what the shop owner and his partner killed Blake over.