In one of the more effective moments of “Spoiler Alert,” the camera does something unexpected and wise: it leaves the room. At the very moment a dining-table conversation becomes unbearably painful, the viewer is moved outside, where we can only watch the characters in shadows through a window, hearing nothing.

We don’t need to hear the words. We know that Kit (Ben Aldridge) and his boyfriend Michael (Jim Parsons) are telling Kit’s parents (a heartbreaking Sally Field and Bill Irwin) that their handsome, charismatic son has a likely fatal disease. We know them all well enough to fill in the blanks.

It’s a moment of admirable restraint, in a film that doesn’t always make that choice. While often deeply moving, “Spoiler Alert,” directed by Michael Showalter and based on the relationship memoir by Michael Ausiello, seems at times unable to decide what kind of film it is, resulting in a number of jarring tonal shifts — particularly one at the very end, which distances us from the characters and their plight just when we’re feeling the most committed to them. At times “Spoiler Alert” feels like an edgy, clever film that plays wittily on the main character’s lifelong obsession with TV. At others, it feels like a more formulaic, holiday-themed tearjerker — the passing years are marked in a Christmas card montage! — that wrings our tears in unsubtle ways.

It’s no crime to be the latter, but the film often reminds us that it seeks to be the former. In any case, bring those Kleenex because you’ll need them. You WILL cry. That much is a given.

We witness the couple’s first meeting at a bar. Michael, a journalist who covers television (Parsons both stars and produces), is hardly one for bars. He’s a “work late, get up early guy” and sticks to Diet Coke. But a colleague has dragged him out, and now he sees Kit across the dance floor, and he’s gorgeous, so, well, that’s that. (Aldridge IS impossibly charming — if you don’t know this actor already you will probably, like me, start randomly Googling “Tell me about Ben Aldridge.” And — spoiler alert! — he’s British and totally nails the American accent.)

Soon after, the two are having dinner and talking about their lives. Truth is, there is little that ostensibly links these two — not their jobs, not their family circumstances, certainly not Michael’s obsession with the Smurfs, though we’re getting ahead of ourselves there. But chemistry works in mysterious ways. Soon they’re at Kit’s apartment, awkwardly hooking up.

Well, awkwardly for Michael, who’s way less comfortable, and explains that he is a FFK (former fat kid) and thus has body issues. In case you can’t quite picture this, we have a running motif where Michael’s youth is depicted as an old-fashioned sitcom — a device that feels clever at first, but wears thin.

Anyway, soon Kit has given Michael closet space in his apartment. When Kit comes down with appendicitis (unrelated to later health issues) his parents insist on coming to New York, and this results in a hilarious attempt by Michael to “de-gay” Kit’s apartment (did we mention? Kit hasn’t come out to his parents yet.) That “Beaches” DVD? Gone. “When Harry Met Sally” too. And much more.

Once at home, though, it takes Mom only a few minutes to figure out that Michael, who somehow knows where the spare sheets are, is more than a casual friend. “I’m gay,” Kit blurts out finally, and the parents are upset — because he hadn’t told them earlier. “We’re actually kind of hip,” says Dad. It’s Field more than anyone in the film who’ll bring you to tears as she navigates love and loss.

More than a decade goes by (punctuated by those Christmas cards) and the couple is having domestic trouble and living separately. But then tragedy strikes, bringing them closer together. We witness the excruciating doctor visits, the differing medical opinions. Soon Kit is starting chemotherapy, and when there’s no bed available, only a chair — a painful situation for Kit — Michael goes into full “Terms of Endearment” mode. “Get my husband a bed!” he bellows at the nurse.

And just when you think it’s all a little TOO Shirley MacLaine, he cops to the joke: “It worked for Shirley MacLaine,” he tells Kit. It’s a sign of the film’s frequent tonal shifts that you weren’t quite sure.

In any case, from here, you’re on your own, dear viewer, because my careful notes devolve into “noooo” and “sooo sad.”

Spoiler alert: Love — romantic, platonic, parental — often comes at a terrible price.

“Spoiler Alert,” a Focus Features release, has been rated PG-13 by the Motion Picture Association of America “for sexual content, drug use and thematic elements.“ Running time: 112 minutes. Two and a half stars out of four.