There’s a great movie lurking somewhere within the nagging clichés of “Death of a Superhero.” Adapted by Anthony McCarten from his 2008 novel, the film should be applauded for its strikingly morbid animation sequences, a winning lead performance from young up-and-comer Thomas Brodie-Sangster and an effectively solemn turn from the normally hilarious Irish comedienne Sharon Horgan.

But with director Ian Fitzgibbon at the helm, the tone shifts uneasily from raucous to bittersweet to disturbing to mawkish. What’s more, since we know the protagonist—an angry, introverted high-schooler dying of cancer—is doomed from the start, it’s hard to get emotionally attached to the central story: the film doesn’t exactly earn its jerked tears.

Brodie-Sangster, registering like a young Malcolm McDowell with his mischievous, feline smile, plays Donald, an adolescent loner who spends most of his time drawing morose comic books. His alter ego is a muscle-bound superhero who’s desired by multitudes of scantily clad women, but too hell-bent on saving the world to give in to their lures. The most wrenching scenes in the movie draw parallels between Donald’s superhuman fantasies and his unflattering reality, in which his better-looking older brother and more outgoing friends are beginning to lose their virginity while he remains shunned by womankind.

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In the movie’s sole fresh twist, Donald is not emboldened by his disease, nor is he particularly brave in the wake of it. He flies into rages, vandalizes his school, rejects the love pouring forth from his supportive parents (Horgan and Michael McElhatton). He even attempts suicide so as to control how he dies. He’s haunted by nightmares in which his own creation, a Death-like comic book villain, tries to murder him. In these sequences, McCarten and Fitzgibbon make Donald’s crippling fear, his inability to differentiate between horrific fantasy and the real fate that awaits him, almost palpable.

Then, the dreaded stock characters emerge and the film turns to fluff. In a shameless swipe from “Good Will Hunting,” Donald is befriended by an equally lonely shrink (Andy Serkis in blandly pensive mode) who’s also a tortured artist, hence he’s the only one who can get through to Donald. In the most groan-inducing contrivance, the therapist hires a prostitute to deflower Donald on his deathbed. Even a third-act plot twist in which Serkis suddenly morphed into his incarnation of Gollum, in “Lord of the Rings,” would be less wrongheaded.

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Slightly more believable, but equally trite, is the romance that develops between Donald and Shelly (Aisling Loftus), a rebellious high-school outcast yet, of course, a sweet-natured soul deep down. Still, it is undoubtedly poignant to watch Donald’s fierce exterior thaw, as he and Shelly grow closer while erasing his profane spray-paint drawing from the classroom window.

At times, “Death of a Superhero” is so spot-on at conveying Donald’s sweet insecurities beneath his steely surface—preparing for a date, for instance, he spray-paints hair onto his bald mirror image—that you wish Fitzgibbon and McCarten didn’t resort to obviousness elsewhere. The film tends to overemphasize its mood changes; heavy metal is heard over the bad-ass comic book sequences, while plaintive folk plays over the misty-eyed ones. We’re always told what we’re supposed to be feeling. And that’s a shame, because the filmmakers clearly possess the intelligence to trust their audience.

“Death of a Superhero,” which premiered at the Tribeca Film Festival last month, is available on Amazon Instant Video; it will be shown at the Seattle International Film Festival tomorrow.