“Kin” opens with starkly beautiful images of abandoned, crumbling Detroit factories. These images seem to promise a blend of visual artistry and dramatic tension that could become a truly satisfying thriller.

That hope lives on for a while after we meet smart, sad-eyed Eli (excellent newcomer Myles Truitt), a 14-year-old who scavenges in those buildings for junk to sell. He hears strange noises in one and happens upon the remains of a mysterious battle, complete with robotic suits of armor, which faintly echo medieval jousting gear, and a boxy metallic weapon that beeps and buzzes and appears to possess big, destructive power. Eli, who seems to have an instinct for operating it, eventually throws the uber gun in a sack and heads home.

All this bodes well at first. The filmmakers are apparently riffing on the legend of King Arthur, of the boy who would rule extracting the sword Excalibur from the stone. But the entire middle section of “Kin” turns that golden introductory idea into dross. Nor does the cool finale make up for the artlessness of all that happens in between.

Cinematographer Larkin Seiple’s fine camera work and Eli’s mystery weapon just don’t keep the thunking, derivative script afloat. Screenwriter Daniel Casey adapted and expanded “Kin” from a 2014 short film, “Bag Man,” by siblings Jonathan and Josh Baker. The brothers, who come from the world of advertising, make their co-directing debut with “Kin.” But somehow they haven’t expanded their source material to fill the space.

Eli is motherless and having issues at school. His adoptive dad (Dennis Quaid) is a gruff, blue-collar guy who sees his son’s factory-scavenging as theft and worries that Eli could be at a danger point in his young life. But Hal can only bark stern advice, and from there, the film falls into one kitchen-sink drama trope after another.

When Eli’s older brother, Jimmy (Jack Reynor of TV’s “Strange Angel”), comes home from a stint in prison, the cliches rain down even harder in dialogue and situations. Jimmy, who is Hal’s biological son with his late wife, needs $60,000 to pay off a local gangster (James Franco, armed, tattooed and chewing on the scenery).

When an attempt to get the money goes south, Jimmy grabs Eli and takes him on an impromptu road trip to Nevada, with stops at strip clubs — tame, PG-13-ish strip clubs — and more illegal activity. A violent scuffle with a club owner (Romano Orzari) and his bouncers triggers the need for Eli to use the weapon, but also alerts Jimmy’s pursuers and the mysterious, perhaps alien, owners of Eli’s gun. The chase is on. The brothers acquire a fellow traveler, Milly (Zoë Kravitz), a club dancer with a heart of gold in a role that feels utterly tacked-on. The climactic shootout, even with the CGI fireworks from Eli’s superweapon, feels like every other cops-and-creeps battle ever made.

Although “Kin” doesn’t cut it, some of its ideas are worthy of further development. The film ends with a hint at a potential sequel, so here’s hoping the Bakers can hone their narrative chops and expand their young hero’s journey to greater effect — if they get a chance.

PG-13. At area theaters. Contains gun violence, intense action, suggestive material, strong language, mature thematic elements and drinking. 102 minutes.