What started as a simple blown stop sign in Dorchester turned into a late-night shocker when Boston police say they pulled over a car and found two teenagers — just 16 and 14 — armed like extras in a mob movie.

It was 2 a.m. when officers spotted the joyriding kids roll straight through Dewey and Dacia Streets. The car screeched to a halt outside a house on Dacia Street, and that’s when police say they saw the backseat squirming — nervous hands moving fast, then freezing the second blue lights hit.

The search turned up a disturbing stash: the boy had a high-capacity magazine stuffed in his jacket, while the girl carried the real prize — a Tec-9 machine pistol with its serial number scratched off, the same weapon once notorious on America’s meanest streets.

The wannabe gangsters were hauled off to juvenile court, facing charges that sound ripped from a crime thriller: illegal possession of a machine gun, illegal possession of a large-capacity feeding device, and carrying a firearm with an obliterated serial number.

No bullets were found, but cops say the fact two kids were cruising Boston with a weapon nicknamed the “street sweeper” is bad enough.