The last episode of “Tom Clancy’s ‘Jack Ryan’” is now streaming on Amazon Prime. It brings the CIA analyst’s story to a satisfying conclusion, with actor John Krasinski stepping away from the titular role after four seasons following a final mission confronting a global conspiracy of Far East terrorists, Mexican drug cartels, and corrupt American politicians.

The first thing viewers will notice is there’s a lot of whispering in “Tom Clancy’s ‘Jack Ryan.’” This could get annoying fast, but the actors make it seem as if they’re concerned about being overheard and daunted by the threat lurking in the shadows.

How they confront these challenges is embodied by Betty Gabriel, as acting CIA director Elizabeth Wright, who invests the audience in her quest for Senate confirmation as she struggles with how many political games she’s willing to play in service of reforming the agency to restore public trust.

The fact that “Tom Clancy’s ‘Jack Ryan’” is a rather clunky title reflects something about what makes the series so unique and satisfying. The audience relates to the daunting challenges our hero faces in ways they couldn’t when seeing the character portrayed by square-jawed Harrison Ford, Ben Affleck, Chris Pine, and Alec Baldwin on the big screen.

That Mr. Krasinski is best known for playing a corporate worker bee in “The Office” lends credibility to his performance as an analyst sifting through intelligence briefings. This Jack Ryan would go unnoticed in a cubicle, which makes putting the fate of the nation in his hands an unsure thing, raising the stakes.

A direct line can be drawn from the Amazon portrayal to what made Tom Clancy’s novels successful, starting with 1982’s “The Hunt for Red October.” Clancy’s realism breathed new life into the genre. Other spies are idealized, and it’s no coincidence that their names are often verbs — Ian Fleming’s James Bond, Vince Flynn’s Mitch Rapp, and Ethan Hunt of the “Mission Impossible” films.

Verbs invoke action, just as TV detectives hew to weapons: Frank Cannon, Thomas Magnum, Anthony Baretta. Clancy not only skipped creating a hero that sounded fictional; he gave him two first names, Jack Ryan, to underscore his anonymity in a massive intelligence bureaucracy with which he’s often at odds.

Lacking the chiseled features of his movie counterparts, Mr. Krasinski spent four seasons developing our hero into someone we identify with, bringing him and his fellow protagonists to life through personal relationships, not wowing us with high-tech gadgets or impossible acts of swashbuckling.

The central relationship is with Mr. Ryan’s girlfriend, Cathy Mueller, portrayed by Abbie Cornish. She’s not trying to be a Bond girl, instead fitting the role of a research physician thrust into the world of espionage. When she says, “Nothing but vipers outside these walls,” it describes to perfection the crucible containing our heroes and anchors us on the inside with them.

Wendell Pierce, as Mr. Ryan’s CIA colleague, James Greer, also looks the part of a world-weary veteran whose marriage and relationship with his son have become casualties of the secret wars he fights, and we can see the miles that life as a field officer has put on Mike November, played by Michael Kelly.

Michael Peña’s Domingo “Ding” Chavez — a veteran of Clancy’s counterterrorism unit, Rainbow Six — asks Mr. Ryan to skip a mission briefing so he can enjoy what may be a final evening with his uncle. The loss of teammates to betrayal drives his character throughout the season as he seeks to visit vengeance on their behalf.

Amazon Studios hasn’t announced a spin-off now that its cornerstone series has concluded, and the Hollywood strike has put any new production on hold. “That’s between Amazon and the Clancy estate,” Mr. Peña told Digital Spy of helming his own series, but he thinks fans want to see it.

If the “Ryanverse” returns minus its titular character, viewers will be back for more adventures, drawn to heroes willing to pay the personal and professional price to keep the American people — who they often reference — safe, and prompting us to hope that there’s more than a handful of real-world counterparts working to do the same for us.