The days of the cradle-to-grave biopic are mostly behind us, with films about historic figures now choosing to narrow in on key events in their lives in order to reveal their souls. It works well in movies like The Queen and Steve Jobs because we are already so familiar with their subjects. But here we have Mank, about Herman J. Mankiewicz, most famous for writing the screenplay for Citizen Kane. We all know Citizen Kane, we all know its director/star Orson Welles, but Mankiewicz isn't exactly a household name. For us to understand him and sympathize with his plights, a straightforward biography would have served (see the more mainstream yet more satisfying Trumbo). What we get instead is a fragmented dive into the spirit of a man we never knew. And by film's end, we still don't know him.

There are several problems with Mank. The biggest is that it takes an irresistible lure – the creation of Citizen Kane! – then shunts it onto the back burner. Although it's ostensibly set in 1940, with our hero (Gary Oldman) laid up in bed with a broken leg after a car accident, dictating the Kane script to an assistant (Lily Collins), Mank spends the bulk of its run time on flashbacks from the previous decade.

Scenes of Mankiewicz's past acquaintance with newspaper publisher William Randolph Hearst (Charles Dance) and Hearst's mistress, actress Marion Davies (Amanda Seyfried), are relevant, since everyone knows that Kane was informed by Hearst's life. But Mank is equally interested in telling us about the 1934 California gubernatorial election, in which Democrat writer/reformer Upton Sinclair faced off against Republican Frank Merrian. The election provides a backdrop for liberal Mankiewicz's various confrontations with conservative studio heads Louis B. Mayer and Irving G. Thalberg – but who on earth thought this would be more intriguing subject matter than the making of Citizen Kane?

Mank screenwriter Jack Fincher thought so, apparently. The director's father, Fincher was a professional journalist who died in 2003. This is his only produced screenplay, and as such there's something amateurish about it, in both the best and worst sense. Its particularity screams "labor of love" for both father and son, which is poignant. But with loquacious dialogue and a slack narrative, Fincher père's script is too enamored with itself, and Fincher fils is too loyal to pare down his late father's florid prose. Funny enough, it's reminiscent of the talky scripts of Mankiewicz's brother Joseph (All About Eve, The Barefoot Contessa, et al). Perhaps it is even a tribute to "Mank" himself, although I know little of his credited work besides Kane.

Sharing to blame for Mank's misses, along with its arcane subject matter and self-involved screenplay, is Gary Oldman in the title role. I am not among those who think Oldman is good in every movie he's in. Reined in, he can be excellent. Let loose, he is simply a ham. Twenty years too old for the part, and bearing no resemblance to Herman J. Mankiewicz whatsoever, Oldman dons a standard "American weisenheimer" accent and gorges himself on Jack Fincher's fatty dialogue. It's a showy, shallow performance, one that ultimately reduces Mankiewicz to the generic "quippy alcoholic writer genius" type we've seen before.

One of the great debates in cinema history is who was the true author of Citizen Kane, since Mankiewicz and Welles shared screenplay credit (and an Oscar). Mank's thesis is clear: Mankiewicz wrote alone, basing his script on his recollections of Hearst, with almost zero involvement from Welles. Yet despite Welles being relegated to a handful of scenes (Tom Burke delivers a credible impersonation) and Oldman chewing every bit of scenery within reach of his teeth, Welles is still more compelling than Mankiewicz. It's just inevitable.

Mank isn't terrible. Erik Messerschmidt's black and white cinematography is gorgeous, it's fun to hear Fincher's regular composers Trent Reznor and Atticus Ross take a stab at old-fashioned orchestral music, and there are some keen insights into 1930s Hollywood studio life. But with its story pointed in the wrong direction, the film never engaged me, and I say this as someone who is unequivocally its target audience.