Five-0 looks into vandalism, arson, and a series of death threats sent to a company whose shark tours are endangering the local shark population. After the third boat of the tour’s fleet is burned three miles offshore, McGarrett (Alex O’Loughlin) and Danny (Scott Caan) head out in search of the boat’s sunken shark cage still holding the body of the owner’s brother. Five-0 searches for the murderer who they are pretty sure the victim managed to wound with a bang stick (an underwater buckshot shotgun used against sharks) as well as investigating the local Kapu surfer gang who attacked the victim a week before his death when the tour began encroaching on their territory and pushing the sharks further inland.
McGarrett and Danny set out to talk with Kawika (Kala Alexander) only to discover that the tour’s owner (Mac Brandt), blaming the Kapu for the burned boats and his brother’s death, has set his house of fire for revenge. Doing their own investigations into the two previous arson attacks, Kono (Grace Park) and Fong (Brian Yang) discover the time-delayed device used to cause the explosion. They trace the method used by an arsonist who denies any involvement in the third attack. When his alibi checks out Five-0 is back at square-one.
Investigating the remains found in a Mako Shark caught by two local fisherman, Max (Masi Oka) discovers a severed arm with buckshot markings inside and DNA that matches the blood found at the crime scene. When they identify the arm as someone tied to a trio of local fishermen who had an ongoing beef with the tour dropping its shark cages in fishing waters they follow the lead to another dead body, a barrel of MDMA, and more questions. Five-0 learns that two couriers from a Taiwanese drug cartel smuggling Ecstasy into the islands were picked up by the DEA two days ago but not before they had dumped their cargo in the ocean.
Danny, Chin (Daniel Dae Kim), and McGarrett manage to catch up to the fishermen sitting on the lucrative but dangerous find that got the victim killed, but they also have to deal with the members of the cartel who have returned for their stolen drugs. In other stories, Chin also gets an unexpected visit form Leilani (Lindsay Price), the prison nurse who helped him survive earlier this season, and McGarrett continues to help Kamekona (Taylor Wily) train for his pilot license, especially after Danny’s declaration that he will agree to fly if the impossible happens, and Kaemkona gets a license. The episode ends with a nice pair of montages as the four members of Five-0 split up into pairs and help both the Kapu and the tour owner with their fire damage before Kamekona takes Danny, Max, and McGarrett up in the first tour of the islands in his helicopter.