IT'S ALWAYS an unnerving but exciting trip when a movie pulls you -- without even offering you snorkel and wet suit -- into that mushy, dangerous quagmire at the bottom of the human heart. You know, where all those powerful impulses lurk like phantoms or flying fish. Waiting, possibly forever, for their summons.

In Dylan Kidd's "P.S.," those impulses lie deep and dormant somewhere inside 39-year-old Louise Harrington (Laura Linney), director of admissions at Columbia University. But when she opens an application from art student F. Scott Feinstadt, you can feel something stirring. The name has enormous significance for Louise. We watch with almost feverish eagerness to understand what this is all about.

He's a quirky kid, this F. Scott (Topher Grace). Fresh and full of beans, he's calling her by her first name in seconds. She summons him to an interview. But judging by her inability to breathe easily, she's summoning him to something quite else.

The truth comes out, little by little. But not before the two of them have already, all but nonverbally, acknowledged their innermost feelings for each other. And followed them.

As if taking mercy on our suspense -- the incredible tension that accompanies this building passion -- Kidd injects much-needed humor. F. Scott looks up in the air, this after they have shared an intense lovemaking session in her home. He looks . . . horrified? Appalled? Terrified? And then he blurts out: "That was [expletive] amazing."

Let's leave it here. This is still in the heady first act of the movie but there are two longer, more excruciating ones to go. It turns out Louise had a boyfriend with the same name (he was Scott), whose tragic death 20 years earlier still haunts her. He was an artist, too. And there's something eerily familiar about a phrase the living F. Scott used in his letter of application. He writes about avoiding a "just add water" life. She's heard that before.

What is she heading into? Who is this person?

Kidd, who made a sensational entrance into indie filmmaking with "Roger Dodger," which starred Campbell Scott as a fascinating romantic trickster, has shown that his debut was no fluke. "P.S.," which Kidd adapted from the Helen Schulman novel, is remarkably assured and sometimes breathtaking for those intimate moments between Louise and F. Scott. And it redounds with tension as Louise starts to investigate F. Scott, and becomes entangled with her best and meddlesome friend Missy (Marcia Gay Harden), who also had history with the departed Scott.

There are further complications in Louise's life: a shattering confession from her former husband, Peter (Gabriel Byrne), about the secret life he has led; and revelations from her brother Sammy (Paul Rudd), a recovering cocaine addict. This relationship with F. Scott is only the beginning . . .

Even though the story ultimately doesn't match the intensity with which it began, the movie's extraordinary for its two main performances. Linney, who emerged like a butterfly from "You Can Count on Me," has flown into an even higher orbit. She's just plain terrific, memorably capturing the mercurial ups and downs of a divorcee, hardly daring to hope for happiness. And as F. Scott, Grace is a force of nature. He creates a thrilling, funny and potentially alarming counterbalance to Linney's Louise. And even if the story closes on a less than satisfactory note, in terms of settling various questions, these two lead us right through to the end titles.

P.S. (R, 105 minutes) -- Contains sexual scenes and obscenity. At Landmark's E Street Cinema.

Laura Linney and Topher Grace give standout performances as a university official and a young art student in "P.S."