There’s a pretty dim nostalgic afterglow at the close of Beyond the Sea, an embarrassingly stiff Kevin Spacey vanity project masquerading as a biographical story of singer Bobby Darin.

The Oscar winner’s co-writing, co-producing and directing is barely competent, let alone his dictatorial decision to star as the teen-idol singer behind hits such as “Splish Splash” and “Mack the Knife.”

One of only two good things going for Beyond the Sea is Spacey’s spectacular singing. And while $8 might seem like a lot for a movie these days, Beyond the Sea is a cheap way to hear Spacey croon in a concert-style setting without paying pesky Ticketmaster fees.

However, the fatal bottom line is that the Oscar winner is far too old to play Darin, at least as straightly as he’d like. The lowest point is an insolent inside jab at critics who might point this out — a bad choice, given it pops up in a scene where Spacey’s sideburns are obviously colored-in.

To respond, did he consider casting, say, Jessica Lange as his wife, fellow teen idol singer Sandra Dee? No, she’s played by the significantly-younger-than-him Kate Bosworth, whose sparkly smile and charm get only as much spotlight as Spacey’s willing to cast her way.

Still, he never looks like anyone other than the 45-year-old man he is despite his caked-on makeup that makes him look like a Dick Tracy heavy. Early fantasy sequences — where a spry Spacey shimmies onscreen in big dance numbers — suggest an imaginative way around the age question, but the idea’s quickly junked. As if one unconvincing Bobby Darin wasn’t enough, there is the annoyingly precocious “Little Bobby” (William Ullrich), who exists in some metaphysical plain to dispense wise advice.

The film descends into melodramatic huffing and puffing, and the house blows in because of a bad foundation — Darin’s life just wasn’t that interesting.

On one hand, it’s commendable for the filmmakers not to grandstand the events. But there is more insight into the inner workings of Sea’s supporting characters than its leading character.

Nina, the woman he believed to be his sister (Caroline Aaron, in the film’s freest and finest performance) was, in actuality, his mother. This revelation wants to lead to a grand mission of self-discovery for Darin, but, in the film’s rare moments of understatement, it says more about Nina.

Darin and Dee’s marital squabbles aim for the gripping level of Ray’s memorable moments, but they are cartoonish and unintentionally funny.

And yes, a childhood illness predicted to be fatal in Darin’s teen years held off for more than two decades. In between scenes serving as a showcase for his chops, Spacey finds room to mention how Darin wasn’t even supposed to be alive to succeed. How convenient that the film shows his reportedly constant oxygen mask use only in the third act, when sickness is supposed to happen.

The end credits reinforce a Darin maxim: “Memories are like moonbeams. We make of them what we want.” Of Darin’s memory, Spacey has made a passionless one-man show about the wrong man. It’s a showcase for Spacey’s pipes, no doubt, but also his pretensions.