An American seaman on a U.S. nuclear submarine dies under mysterious circumstances during an AUKUS ceremony in Sydney Harbour.

Forensic Pathologist Dr. Rosie Penrose examined the seaman's body.

He's revealing his findings to Sgt. Jim "JD" Dempsey and his AFP contingent when NCIS Special Agent Michelle Mackey bursts in claiming jurisdiction.

After some jousting, JD and Mackey get called to meet with the Australian Foreign Minister. who orders them to work together under Mackey's leadership, which JD grudgingly accepts.

The victim, Petty Officer Flynn, liked to drink and brawl. Blue discovered he'd had a date the night before.

Flynn had gotten into a fight at a bar. Blue uses borrowed tech to identify with whom he fought.

That man, Evan Gerber, almost gets away from JD but Mackey tackles him.

Rosie clears him, telling JD and Mackey that radioactivity killed Flynn.

Col. Richard Rankin, a DOD official, confiscates Flynn's body from Rosie, which blindsides Mackey.

Lt. O'Leary is missing from the sub.

Rankin and the foreign minister stonewall Mackey and JD.

Then, after hearing a gunshot, they discover O'Leary shot in his motel room.

JD feels the "suicide" was staged.

To keep the body from being snatched, Rosie takes it to a naval storehouse.

O'Leary had truth serum in his blood.

Blue establishes that Flynn was killed by Polonium 210, not by radium like was used on the sub.

The team interrupts a group of Russians who have infiltrated the vacated ship.

Outgunned, they're chased into a shipyard.

The Russians' leader captures DeShawn. 

Mackey pretends she's reaching for a gun and gets shot, allowing JD to kill the Russian. Fortunately, she's wearing a bulletproof vest.

The rest of the Russians are escaping by boat. 

Mackey borrows a chopper from the aircraft carrier and gives chase. But the boat is rigged to explode when they get close.

They had broken up a sleeper cell although the blonde woman likely escaped.

Mackey and JD expect to be fired but instead NCIS: Sydney is created.