Frogmen steal an Army shipment of plastic explosives. The ID number on the power boat in which they escape is spotted and relayed to Five-0, which traces the boat to one Jim Ryan (Lee Stetson). When Ryan spots Five-0 at his house, he alerts his contact, Foley (Tar), who promises to help him out.
Meanwhile, Casey Fogarty (Elayne Heilveil), the daughter of a sentimental millionaire, arrives in Hawaii carrying a briefcase full of money to help the Irish cause. Calling the phone number she has been given, a man answers the phone and tells her to meet him near Hanauma Bay. When she asks how she'll recognize him, he says, "Are you Catholic?" When she replies yes, he says she'll have no problem. Indeed she doesn't. At the Blow Hole overlook, she meets a man in a clerical collar who introduces himself as Father Daniel Costigan (Steven Boyd).
Word has come from intelligence circles that the explosives have been stolen for use by an IRA splinter group. This draws Danny's attention to a newspaper article about newly arrived Casey Fogarty, who is described as a radical student activist. McGarrett drops by her hotel for a chat but gets nowhere. After he leaves, she goes to see Costigan. He has dropped his pretense of being a priest. The two of them are attracted to each other. When she volunteers to enlist in "the cause," she uses her father's line, "Ireland needs heroes." Costigan darkly replies, "There are no heroes. Only desperate men and fools .... and graves. Too many graves."
Foley has coolly passed on Ryan's whereabouts to Costigan, who says, "I like doing business with professionals. They have no ethics, no principles, and no moral standards. So one always knows what to expect from them." With Casey present, he calls Ryan's number and hangs up when Ryan answers. Ryan panics and calls McGarrett for protection. Unfortunately, Costigan arrives first in priestly garb and terminates Ryan.
The Five-0 team has been following up on all the visitors from Ireland who have showed up in the last few weeks. Only one name remains, Father Daniel Costigan, who is said to be on leave from his parish to visit Catholic charities in Hawaii. McGarrett visits him at St. Claire's and is suspicious enough to put a tail on him.
Costigan evades the tail by dressing in aloha wear and having Casey pick him up in a borrowed van. She drives him to Foley's place. Foley, it seems, has started to blackmail Costigan as soon as news of Ryan's murder came out. Costigan gives Foley a case full of money, which explodes as soon as Costigan and Casey drive away. Although Casey believed Costigan when he said he had nothing to do with Ryan's death, she now says, "You lied to me." He replies, "It's an occupational hazard." Murder, he says, is a "tactical necessity." When the cause is in danger, "There's no room for questions -- or conscience."
Packaged as toys, the stolen explosives are loaded onto an outbound freighter. Casey eagerly asks to go with Costigan. He appears to agree but asks her to return the van so that there'll be no questions asked. He then boards the boat without her.
Five-0 has investigated the explosion at Foley's place and found a scrap of paper that leads them to the warehouse where Casey is dropping off the van. Even under pressure, she won't tell McGarrett where the explosives are. Parroting Costigan's words, she tells McGarrett that the deaths resulting from using those explosives are a tactical necessity. Outraged, McGarrett shows her pictures of a blast in Ireland that left children dead and maimed. "It's insanity," he barks at her. She breaks down and tells him the truth.
McGarrett races to the dock, flags down the boat and arrests Costigan who, as they are putting him into the squad car, winks at Casey and says, "Up the rebels!" To which Steve quietly replies, "And God help the children."
NOTE: This stunning episode ranks near the top of my pantheon of Hawaii Five-0 greats. Besides a timely and convincing story, what made it so special were stunning performances by Steven Boyd, Elayne Heilveil and Jack Lord. Boyd's ability to communicate Irish come-hither, coiled power and world weariness, made this one of his finest performances in any medium. And who can forget the dewy-eyed eagerness and youthful hormonal charge that Heilveil brought to her scenes with him. Wowl As for Jack Lord, his interaction with both of them lifted those scenes to a level of intensity seldom achieved during the run of the show.