There have been plenty of movies revolving around high school reunions (among them, 1982's "National Lampoon's Class Reunion," 1986's "Peggy Sue Got Married," 1996's "Beautiful Girls," 1997's "Romy and Michele's High School Reunion," that same year's "Grosse Pointe Blank," 2012's "10 Years," and 2012's "American Reunion"), but few have had quite as forlorn a heart as first-time directors Andrew Mogel and Jarrad Paul's "The D Train." Oh, and it's a comedy, albeit a progressively dark one surfing on a wave of audience discomfort and concern for its sad-sack protagonist, small-town accountant Dan Landsman (Jack Black). Arrestingly written by Mogel and Paul, the film shatters expectations at around the 30-minute mark and keeps surprising thereafter, refusing to contrive an easy way out for one character who no longer seems to know who he is and another who plays the part of a hotshot while masking his own insecurities.

The 20-year reunion of Grant Barklidge High's Class of 1994 is fast approaching, and committee member Dan is still struggling for acceptance with the former peers he has only vaguely kept in touch. If making friends has never been his strong suit, he does have supportive wife Stacey (Kathryn Hahn), sensitive 14-year-old son Zach (Russell Posner), and a new baby at home. When Dan stumbles upon a familiar face late one night in a sunscreen commercial, he is more than a little excited to discover it is Oliver Lawless (James Marsden), one of the most popular kids from his high school class. Hoping to finally garner the respect of the committee and inspire more people to attend the reunion, Dan concocts a bogus excuse for a business trip so that he can head to Los Angeles to seek out his "celebrity" classmate and convince him to attend. When Dan meets Oliver for drinks, he tries to play it cool but cannot help but be in awe of this handsome, seemingly self-assured man who knew what he wanted out of life and wasn't about to let his Pittsburgh hometown keep him from it. The two of them hit it off better than Dan could possibly anticipate, but he is left desperately confused by what ultimately transpires between them before his time on the west coast comes to a close.

Because it is so difficult to discuss "The D Train" without giving away key plot points, those readers who haven't yet seen it would do best to stop here and return only after they have. The major turning point of the narrative—the new friendship between Dan and Oliver that suddenly, if fleetingly, becomes physical—is handled by writer-directors Mogel and Paul with a refreshing forthrightness and, thankfully, not as a punchline or a point of ridicule. The more experienced, sexually free Oliver doesn't think anything of his actions, but for Dan their night of indiscretion is earth-shaking. When he returns home, he tries to pick up with life as normal, but the lies he has already spun to Stacey and his gullible, old-fashioned boss, Bill (Jeffrey Tambor), only pile up from there. And, when Oliver decides to attend the reunion after all and comes to stay with Dan and his family, Dan's emotions—a trifecta of guilt, jealousy and turmoil—prove impossible for him to bottle.

What is so fascinating and almost novel about the picture isn't that Jack Black's (2012's "Bernie") Dan and James Marsden's (2014's "The Best of Me") Oliver sleep together, but why they do, and how they deal with it after the fact. For casual, irresponsible playboy Oliver, he sees it as a drug- and alcohol-fueled one-night fling. For Dan, who isn't necessarily gay, his serious case of idol worship gets the best of him as this person he intensely looks up to shows him the interest and validation he's been craving for so long. The adulthood he has built for himself is a reasonably happy one, but he has made a deceptive, self-destructive path for himself, one that could end his marriage and cause him to lose his job. Worse still, shrugging off this illicit intimate encounter is impossible for him to do without inevitable repercussions. He no longer knows himself or what his future holds, and Oliver's reappearance only salts his personal crises.

Whether a calculated career move or just a matter of circumstance and coincidence, Jack Black's turn toward smaller, more adventurous projects in recent years has suited him well. He is capable of far more than just being a funny guy in broad studio comedies, and has possibly never been better or had such a complicated character to tackle as he does here with Dan Landsman. Dan is a troubled guy who tries too hard in order to compensate for a loneliness his wife and kids cannot fulfill. If he has an occasionally overbearing nature and makes choices that aren't always sensible, he has a good soul. More than anything, he wants to be liked, and the inner struggle Black movingly brings to the screen is riveting to watch play out. James Marsden is superb as Oliver Lawless, a struggling actor who Dan puts on a pedestal, unaware that he is a virtual nobody in Hollywood. It would have been easy for Oliver to have been written as an aloof, one-note stereotype, but there is more to him, too, than meets the eye. In scenes like the early one where Dan spots Dermot Mulroney at a club and Oliver acts like he knows the well-known star in order to impress him, it is obvious that he is riddled with his own brand of self-doubt. In the second half of the film, he doesn't readily own up to the mess he's made, but he does care enough about Dan to not just write him off. As Stacey, Kathryn Hahn (2014's "This Is Where I Leave You") also is given more to work with than just the thankless part of a nagging wife. While the story requires that she be in the dark about what is going on for a large portion of the running time, Hahn refuses to portray her as a fool. When the truth comes out, her reaction is honest and understated.

"The D Train" is more brazenly entertaining than it may sound. As sticky and awkward as the situations get, and as much pain as Dan goes through, the film is not about his defeat, but about his path toward accepting himself. Uncompromising until an ending that works itself out a bit too easily and neatly, the picture buzzes along with the promise that anything can happen and the conventional rules of mainstream "bromance" comedies (a term that is cringe-worthy to a fault) do not apply. Complemented by Andrew Dost's sweepingly nostalgic score—one that sounds as if the soundtrack from a long-lost John Hughes movie has just been uncovered—"The D Train" makes a potent, empathetic mark while providing concrete proof that Jack Black has what it takes to have a second life as a character actor of immeasurable depth and compassion.