Kevin Spacey scratches an old itch with his singing portrayal of rocker-cum-crooner Bobby Darin in “Beyond the Sea,” and he can now envision himself as Darin until the cows come home.

Better him than me, or the unsuspecting cows.

I saw Darin perform at his peak in the early “60s, and though Spacey has a strong voice and does a fair imitation of Darin’s finger-snapping nightclub style, it eludes me why anyone would want to see an actor in his mid-40s play a twentysomething heartthrob from a bygone era.

Darin fans will be trading down musically, and those who never knew of the boy wonder who died at 37 in 1973 will wonder why a movie was made about him – especially this movie, which Spacey also directed.

Give the actor credit for a well-developed ego, and make room for it. It wasn’t enough to reenact the short, unhappy life of his singing idol and perform every note on the soundtrack. He had to attempt an imitation of Bob Fosse’s masterful “All That Jazz” at the same time.

“Beyond the Sea,” written by Louis Colick (“October Sky”), clumsily merges fiction and reality, biography and musical fantasy, and breaks the fourth wall in a way that allows Spacey to lamely address his own miscasting.

“He’s too old to play the part,” someone says of the older Darin’s playing the younger Darin in a movie within the movie.

“Well, he was born to play the part,” says another.

The audacity of that exchange is stunning, and if Spacey thought it would cover the conceit of his playing a man half his age, he was wrong. The long center section of the film, dealing with Darin’s relentless pursuit of screen sweetheart and future wife Sandra Dee (Kate Bosworth) – he was 24, she was 16 when they met on the Rome set of “Come September” – is painful to watch.

The fact that Dee’s mother (Greta Scacchi, who’s a year younger than Spacey) doesn’t lock her in her trailer and call the polizia seems a clear case of child neglect.

The awkwardness of the film’s structure and its inept theatrics diminish the real drama in Darin’s life. Born in the Bronx to an impoverished family, he contracted rheumatic fever as a child and was left with a heart condition doctors said would kill him before his 15th birthday. They were 22 years off.

The death sentence motivated him to accelerate his career and to consume as much life as possible.

It’s hard to execute a ticking-clock premise when there is no deadline; in fact, Darin might be alive today if it weren’t for complications from relatively routine surgery to fix a heart valve.

But Spacey and Colick construct their story around that premise as they head toward an “All That Jazz” finale.

A subplot about Darin’s relationships with a grandmother (Brenda Blethyn) he thought was his mother and the mother (Caroline Aaron) he thought was his sister is blown into a melodramatic cartoon in which the grown Darin is often seen talking to his boyhood self (William Ullrich).

But forget all that. The sole attraction here is Spacey’s singing, and even if his voice is only about 80 percent of Darin’s, it’s pretty remarkable.

If you’re curious about his impersonation, you can buy the soundtrack. Or you could just buy one of Darin’s albums. In fact, “Beyond the Sea” was my favorite.

BEYOND THE SEA

1 1/2 stars

With Kevin Spacey, Kate Bosworth, John Goodman. Directed by Spacey (2:01). Rated PG-13 for strong language and a scene of sensuality.