A shark interrupts a seniors swim class and leaves behind an arm with Navy equipment attached to it.

Rosie samples the product of the office's new espresso machine then DeShawn shows off his new gun.

With no walk-up victims of shark attacks to be found at hospitals, Mackey pivots to x-raying the shark.

Mackey works from the assumption that the SEAL is still alive and checks with a unit training in Sydney to see if anyone is missing. But no SEAL is.

The shark x-ray doesn't turn up a diver on board.

The dive computer which Blue hacks fails to provide much useful intel.

Blue and Mackey continue not to connect.

Rosie captures fingerprints for Phillp Coleman, a retired petty officer and SEAL 

Mackey and J.D. question Coleman's dive-shop coworkers, who figured he was just on a bender.

Blue determines Coleman was diving for an unmanned underwater vehicle, which had disappeared by this morning. It was likely privately owned.

Next, they look into Coleman's dive buddies.

A one-armed body washes up on a beach.

Mackey chases after a man acting suspiciously at the beach where Coleman's body is discovered but he drives away.

D and Evie pose as an engaged couple looking to get married on a boat.

D's gun bulge gives him away to ex-SEAL Hammer.

Evie gets caught inspecting dive tanks but overcomes Oyster and rescues D.

She had found they were recovering smuggled drugs. But none claimed to know about the missing drone.

Rosie figures out that someone killed Coleman with a sharp implement to his brain before feeding him to a shark.

Blue is planning her own farewell party.

Based on a comment by Blue, Mackey locates the missing UUV in a shark breeding cave. But the data drive is missing from this spy drone.

Oyster killed Coleman hoping to cash in on the surveillance footage, which the other two divers didn't know.

Evie recovers the footage from where Oyster hid it in an office fish tank.

Now they have to determine who hired the divers for what looks like espionage.

Hammer and Nails meet up with their client Mars, who framed them with the drugs. Then NCIS busts them all.

Col. Rankin takes possession of Mars, a DOD employee who had been leaking intel.

Blue tells Mackey the MDMA they recovered came from a DEA bust and was supposedly destroyed six months ago.

They confront Rankin and demand he take care of Hammer and Nails or they'll leak about a CIA drone being in Sydney Harbor.

The team surprises Blue with the news that she'd continue to be a member of NCIS: Sydney.