Like salmon that swim upstream to spawn, there must be some innate quality in young boys that causes them to build forts in their back yards out of old boxes. According to “Millions,” it’s seemingly as common a practice in Britain as it is in America.

So begins Danny Boyle’s whimsical caper, with 7-year-old Damian Cunningham (Alex Etel) constructing a cardboard haven near the train tracks behind his new home. Damian, his upbeat dad (James Nesbitt) and 9-year-old brother Anthony (Lewis McGibbon) have relocated to the new neighborhood after the death of the kids’ mother, and the family is making the usual adjustments.

“We just moved here; our mum’s dead,” Anthony matter-of-factly tells a shopkeeper as a way to get a free treat.

Soon, however, the boys need to engage in a different kind of con when Damian’s fort is cannonballed by a black duffel bag with a Nike swoosh. The satchel is filled with hundreds of thousands of English pounds. (“Hundreds of Thousands” apparently didn’t make quite as fetching a title as “Millions.”)

The ultra-religious — or at least Catholic trivia-obsessed — younger Cunningham at first believes it’s a gift from God. But he learns it’s really the product of a major robbery, and the thug who flung it from a speeding train begins lurking around the vicinity hoping to reclaim it.

But here’s the kicker: By the end of the week, Britain will switch totally to the Euro. Any pounds not exchanged will become worthless.

Part comedy and part morality play, “Millions” is an odd hybrid that never quite goes where the viewer thinks it should. Most of the humor (and tension) comes from the fact that Damian wants to spend the money to help those less fortunate, while his brother insists they keep things secret and use it for themselves. This leads to plenty of awkward moments when Damian tries to solve problems by anonymously delivering thick wads of cash.

And both brothers find it’s not exactly easy spending piles of currency without drawing attention when you’re a kid.

Alex Etel, left, and Lewis McGibbon portray English boys who stumble upon a bag full of money in filmmaker Danny Boyle's Millions.

“I was going to give it to the poor,” Damian says after much distress. “But it was really hard.”

Director Boyle (“28 Days Later,” “Trainspotting”) avoids the grotesque images and ultra-violence of his previous films, but he heaps on plenty of weirdness. Damian often holds conversations with patron saints such as St. Joseph and St. Clare, who advise him about his dilemma while digital halos hover above their heads. It’s one of many surreal elements Boyle and writer Frank Cottrell Boyce (“24 Hour Party People”) insert into the narrative.

Yet whenever the movie really starts cooking along — as during a tense sequence during a school’s nativity play — the fantasy element comes in to let the characters off the hook. What makes “Millions” quirky is also what prevents it from genuinely immersing the viewer in the story the way Sam Raimi’s comparable “A Simple Plan” does. Or Boyle’s first film, “Shallow Grave,” for that matter, which shares some striking similarities.

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The ending is particularly frustrating in this respect. It forces the picture to be regarded more as a parable than a character-driven story.

Fortunately, newcomer Etel is there to help bail things out.

The freckle-faced lad is so convincing that we believe his dedication to help the poor. We believe his obsessions with dead saints, even when he’s enjoying casual chitchats with them. Most importantly, we believe his moral grounding. What he holds true may be simplistic, but it’s usually the “right thing to do” in the midst of tremendous temptation to behave otherwise.

Alex Etel, left, confers with Enzo Cilenti as St. Francis during a fantasy sequence in Millions.

Even though Boyle tries everything he can to distract viewers with cinematic figments and pizazz, Etel keeps the film fortified with realism.