It’s nice to see Hugh Bonneville truly slip the tuxedoed bonds of “Downton Abbey” and take on a real rotter for the change.

In the thriller “I Came By,” he may be posh and entitled, if not titled. But there’s little that’s soft about this just-resigned-judge who’s run afoul of pranksters who stumbled into his “secret.”

George MacKay of “1917” stars as Toby, a tag-happy graffiti artist who veers between righteous and self-righteous in his quest for attention and rough justice. Toby and his mate Jay (Brit bit player Percelle Ascott, getting a nice break) break into the houses of the rich and rattlecan-paint statement “tags” in prominent places.

“I Came By” is their signature and their message. To Toby, they’re “showing them we can GET to them.” Maybe they’re giving other graffiti artists a bad name, but the fit, anarchistic Toby is dedicated to the cause…and little else.

His psychotherapist mum (Kelly Macdonald) is at a loss and fails in every effort to make him shape up and find direction. And when Jay and his girlfriend Naz (Varada Sethu) get pregnant, his partner in political crime checks out.

But not before Jay gained access to a pricey home owned by a former judge (Bonneville) and gave Toby the idea of tagging him. Jay’s take was that the judge supported the right causes and seemed like a righteous chap. Toby, determined to carry on without him, does not care.

That’s how the punk discovers the judge’s secret, disappears and puts everyone he knows in jeopardy in the process.

Much of the film has Macdonald playing an out-of-the-loop, guilt-ridden and desperate mother trying to find her son, or what happened to him. Jay might help her, but the just-clever-enough mother Liz has a bit of her son’s rashness and recklessness. And Jay gives up what he knows far too reluctantly for Liz’s needs.

Bonneville brings on the slimmed-down and sinister as this well-connected menace, saying the right things, playing the angles, gambling that he’ll know how to handle and deflect the police when the chips are down.

The plot points and plot devices aren’t the most original in this Babak Anvari film. But I love the graffit-as-protest hook. The cast is spot-on, the action beats visceral and desperate. And the British-Iranian filmmaker works lots of inclusive touches in around the edges — the judge’s immigration connections, Naz and Jay’s interracial relationship, which her parents condemn.

It’s similar to far too many recent films to shock and impress, but “I Came By” is one of the tighter thrillers Netflix has put its money behind. And MacKay, Bonneville, Macdonald and Ascott remind us in this genre, it’s what you spend on acting talent is what matters the most.

Rating: TV-MA, violence, profanity

Cast: Tom MacKay, Hugh Bonneville, Percelle Ascott, Varada Sethu and Kelly Macdonald

Credits: Directed by Babak Anvari, scripted by Babak Anvari and Namsi Khan. A Netflix release.

Running time: 1:50