Michael Emerson, Taraji P. Henson, Jim Caviezel

Jim Caviezel and Michael Emerson have already had a long day. They're shooting the first scene of a new Person of Interest episode featuring their crimefighting alter egos, Reese and Finch, inside the latter's sepulchral lair before the production heads out for another exhausting location shoot.

This after having spent the just-barely-morning hours wrapping tonight's episode (9/8c, CBS) on Manhattan's Upper East Side. It's a game-changing story line that will see POI segueing into a strange(r) new world as costar Taraji P. Henson's Detective Carter, the pair's erstwhile nemesis, allies herself with them.

It's a neat swerve, one that was accomplished during a Henson-centric trilogy of episodes beginning with December 8's "Get Carter," in which Reese saves the detective from an assassin's bullet. In December 15's "Number Crunch," Carter's repeated intersections with Reese and Finch resulted in an internal affairs investigation, and she met a shady CIA operator named Snow (Michael Kelly) who's after Reese, allegedly because he wants to prevent him from causing any more chaos.

As Emerson puts it, "We get in over our heads and need her help." But it's mostly Carter's increasing identification with Reese that sets the stage, a point driven home in a flashback revealing that she, like Reese, was a soldier who became disillusioned by some of her government's war-on-terror tactics. "She recognizes him as one of her own," Henson says of Reese, "and a bond develops."

It's a plot turn that the show's brain trust has been hankering to reach. "[We] always felt that we were heading in this direction," says POI cocreator and exec producer Jonathan Nolan. "It's a happy development," agrees Emerson. "It complicates the show and it makes them an odd group of musketeers." It's certainly not happening too soon for Henson: "I think I'd gouge my eyes out if I kept chasing them for 22 episodes!"

Henson is pleased but not surprised with the development. "That's why I took the job," says the Oscar nominee (for her role in 2008's The Curious Case of Benjamin Button). "I've done television before, and it's really not my favorite. But, when I got the call from [Nolan], he explained that it would be a three-person gig."

That triad gets an interesting workout in tonight's episode, "Super," as Reese, recovering from a gunshot wound, finds himself taking on a more Finch-like role in quarterbacking the team's investigation into a creepy building superintendent (Dexter's David Zayas).

But while Carter is an ally, she's not an equal partner, owing to Finch and Reese's obsession with secrecy. "Finch worries about the world knowing about the machine, so I think he must surely have misgivings," says Emerson. "With Carter," adds Caviezel, "everything is on a need-to-know basis."

Making their new alliance even trickier: Neither Carter nor her fellow NYPD detective Fusco (Kevin Chapman) know that the other is in cahoots with the dynamic duo. "It's a fun driver of tension," says Nolan of that decision. Besides, says Caviezel, "It serves as a check-and-balance. Fusco could turn on a dime. Reese likes him, but that doesn't mean he can't kill him."

For Nolan, Reese's black-ops background will come in mighty handy as he and Finch's enterprise gains more moving parts: "We're taking that tension from the spy world and injecting it into a police procedural, hopefully with really cool results." Mission accomplished.

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