“A Patient Man” is a cagey tale of loss, grief, guilt and revenge.

It does such a swell job of hiding its secrets I’m loathe to give any of them away. Thrillers that keep you guessing, sometimes correctly, sometimes sending you up a blind alley, are rare.

That’s also why it’s a shame when the picture is let down by a flat performance at its heart, however justified a “poker-faced” turn might be to the actor making that choice.

Improv comic Jonathan Mangum  has the title role. Tom is a buttoned-down guy using a bike and LA’s light rail system to get to work. But he’s not doing it to save money or save the environment. He survived a terrible car wreck, and he has lingering issues from that accident, and not just physical ones like the knee brace he wears.

“Are you afraid to get back in a car?” his shrink (Kelsey Scott) wants to know.

“Something like that, yeah.”

They’re very understanding at work, full of “Sometimes, bad things just happen” platitudes because who can think of anything better to say at a time like this?

He bikes to work. He does his boring job, advising clients in logistics at a consulting firm. He bikes home. He drinks. And the next day, he does it again.

And he tries to “feel better” by telling this all to a therapist.

“Like pills and television, my job makes time pass strangely.”

But he has lots of flashbacks. Odd automotive things trigger him. And he’s asking odd questions of his lawyer/friend (Elaine Loh). He’s curious about the person who ran that stop sign and T-boned his car.

He may say he is just “looking at what is directly in front of me” just to cope, we know there’s more going on. Will he start over, joining a colleague for “Night Rider” group bicycle rides through the city? Has he made a new friend on the train?

Is he falling for his therapist? And does he have a prayer of getting “the big promotion” at work?

Mangum, who got his start with Wayne Brady at Orlando’s famed SAK Comedy Lab, plays this guy so deadpan as to be sleep inducing. Flashes of sarcasm or wit play just as flatly as everything else coming out of his/Tom’s mouth.

As we plumb the unfolding story for clues, catching the fake-name offered here, the “big reveals” about the accident and those involved, we’re hard pressed to connect with the guy or care.

Every choice made about Tom is valid. Great loss is deathly deflating. Finding the will to carry on does not make you the life of the party.

But brother, it’s a movie. You’ve got to give us something, some spark of dark humor, some sinister subtext, SOMEthing.

Veteran TV editor Kevin Ward has cooked up a smart script for his feature directing debut. Too bad he and Mangum couldn’t come up with a more interesting, charismatic approach to the character. Or was he the only bicyclist/actor available in LA?

MPAA Rating: unrated, alcohol abuse, profanity, violent car accident

Cast: Jonathan Mangum, Kelsey Scott, Tate Ellington, Elaine Loh and David Jahn.

Credits: Written and directed by Kevin Ward. A Commuter release.

Running time: 1:33