In any decent dog film, it’s practically a given that the featured canine will die at the end.

But how about a pooch-picture where death comes first? And where it’s actually the owner who passes?

That would be “The Friend,” a dog movie that’s not just about dogs.

In fact, with yet another stunning lead performance from Naomi Watts, this sensational film starts out slow and gentle, then builds to an emotional flood-tide in its third act; the powerhouse plot eventually addresses grief, philandering, loyalty, suicide, fortitude, healing — and yes, lasting love with man’s best friend.

Or woman’s.

Watts plays a Manhattan novelist whose close pal and mentor — a womanizing older writer played by Bill Murray — rather unexpectedly takes his own life. Meeting with Walter’s third wife and grieving widow, Iris learns that the late author wanted her to have his recently rescued dog — a sad-faced Great Dane named Apollo. But as Iris is still grieving herself — and struggling with her latest book — she doesn’t have the time, energy or experience (or the dog-friendly apartment!) to deal with this gigantic new roommate.

That probably doesn’t sound like enough story for a two-hour film. But while some of its early scenes feel underwritten, “The Friend” is never boring; before too long, we’re thoroughly captivated by Iris’s odyssey — and we can’t stand the thought that she might have to give Apollo up to keep her rent-controlled digs.

Much of this enchantment is due to Watts, whom I raved about in last week’s Webb, extolling her impressive filmography: Mulholland Drive, Penguin Bloom, The Ring, Eastern Promises, King Kong, Birdman, The Impossible — and of course St. Vincent, in which she likewise worked with Murray. One wonders how long this 56-year-old veteran can keep charting new territory and becoming ever more accomplished. Which is my way of saying, she’s better than ever in “The Friend.”

In its amazing final scenes, the script moves into decidedly non-dog territory, highlighted by a fascinating “metafiction” sequence where Iris pens a post-death exchange between herself and Walter. It’s a brilliantly acted and written moment, with trenchant reflections on suicide that pretty much every human being needs to hear.

Murray is terrific, starting out as his usual lovable wise-guy and then moving into a self-centered sad-sack who never considered how much his suicide was going to hurt the women in his life.

Or his dog.

Speaking of which: If it were possible for a Great Dane to snag an Oscar for Best Supporting Actor, this might be the film to do it. Apollo — played by a well-coached dog named “Bing” — is simply mesmerizing; he has an even sadder-sack mug than Murray — one in which we can read all the emotions we expect him to feel; and one that also reflects the feelings of those around him.

Because, you see, it turns out that in his relationship with Iris, Apollo is not the only grieving creature who gets comfort and healing….

“The Friend” is the sort of quiet, reflective movie that doesn’t draw huge crowds; so get out to The District and see it soon — or put it at the top of your streaming-list.

This one shouldn’t have to sit up and beg.