Please don’t miss the first five minutes of “The Discovery.”

To be clear: Please don’t miss the first five minutes of ANY movie — give yourself plenty of time to load up on snacks and all that — but let’s be real, if the movie in question is about flying cars or some such thing, you might be able to catch up with developments if you stroll in just a tad late.

Not so with “The Discovery.” You need to be strapped in and focused for director and co-writer Charlie McDowell’s ambitious, unnerving, slightly loopy and beautifully ambivalent gem, which only tackles the question: How would people react if there was absolute proof of an afterlife?

The first thing we see in “The Discovery” is a close-up of Robert Redford’s magnificent, craggy, Movie Mount Rushmore face. Redford’s Dr. Thomas Harbor is a renowned scientist who has discovered proof there is some form of life after death — and now he is about to grant his first interview, to a television journalist (the invaluable Mary Steenburgen.)

“Don’t you think your discovery was just too dangerous to share with the world?” asks the TV journalist.

Dr. Harbor’s response, in part: “How can you keep a discovery so vital to our existence a secret?”

I’ll not say what happens from there, other than it sets in motion a fascinating albeit sometimes convoluted journey that reminded me a little of “Eternal Sunshine of the Spotless Mind” and a little of “Vanilla Sky” — though in many instances, it felt like its own original and new and fresh creation.

After that initial, shattering scene, we cut to sometime in the future, as Thomas’ neurologist son Will (Jason Segel) is on a ferry bound for the remote island estate where his estranged father has retreated from the world. (If you don’t think Jason Segel from “How I Met Your Mother” and “Forgetting Sarah Marshall” can convincingly play Robert Redford’s son who is a neurologist, check out his work as the brilliant and troubled writer David Foster Wallace in “The End of the Tour” and get back to me.)

As we learn from a newscast blaring from a TV set on the boat, Thomas’ discovery has led to millions of people around the world committing suicide, apparently because their lives are so bleak, they’re willing to embrace a second chance via whatever awaits in the afterlife, whether it be a reincarnation of sorts or an existence on another plane.

The only other person on the boat is Isla (Rooney Mara), a feisty platinum blonde. When Will says she looks familiar and asks if they’ve met before, she sizes him up, says it’s quite possible — but he doesn’t have the kind of face people remember. (McDowell and co-writer Justin Lader expertly sprinkle in sharp one-liners throughout the film, providing wicked chuckles at just the right moments.)

Isla is on her way to the island for reasons we will not reveal. Suffice to say she winds up partnering with Will on a number of levels.

Jesse Plemons (“Friday Night Lights,” “Breaking Bad”) provides his usual memorable work as Will’s wild-card brother, who worships their father and has bought into his project — that project being an almost cult-like retreat where Thomas attempts to wean the guests from suicidal thoughts. (The “guests” wear different colored jumpsuits, perform chores and gather in ritualistic fashion. So yeah, cult-like.)

Redford gives one of his most interesting and offbeat performances in recent years as Thomas, who’s part philosopher, part mad scientist, part grieving husband, part father (figure) to not only his sons, but also to his unwanted disciples. (For such a brilliant man who has made such a huge discovery, Dr. Harbor appears to have not thought things all the way through.)

“The Discovery” doesn’t try to wow us with special effects and scientific mumbo-jumbo. The machine providing a glimpse into the alleged afterlife has a decidedly retro, 1990s-movie look. Journeys of the subconscious into the next world are portrayed as memories crossed with the kind of dream you’re having just before you wake up, when you’ve got one foot in both worlds.

Until the very end, we’re kept guessing — not only about the true nature of the afterlife, but about the motives and the secrets held by a number of key characters. The biggest “answer” might leave some viewers frustrated and/or confused; I’ll confess I felt tinges of both.

But just tinges. Primarily, I was impressed by the universally fine performances, and by the ambitious and mostly successful attempt by Charlie McDowell to take us into a different world.

So please. Get there in time for the first five minutes. And budget a lot more time than that for the after-movie discussion.

★★★1⁄2

Netflix presents a film directed by Charlie McDowell and written by McDowell and Justin Lader. No MPAA rating. Running time: 101 minutes. Debuts Friday on Netflix.