By day, he’s a suit-clad, 30-something environmental lawyer. He sits at a desk, reads memos, makes small talk at the water cooler.

But by night, he’s Agente Punto Final (Spanish for “period”), savior of Ecuadorian letters, copy editor to the masses. Armed with stencils, a few cans of spray paint and a passion for correct grammar, he has one mission:

“We will not break, we will not give up until my city is free of spelling mistakes on the walls,” he said.

Since last November, Punto Final and his compatriots, Agente X and Agente Diéresis (the Spanish term for an umlaut), have been working to correct the spelling and grammatical errors of Quito’s graffiti artists. All three are anonymous (charges of vandalism in Quito can result in a fine and up to five days in jail, according to Punto Final) and they take stealth very seriously. Each operation begins with a reconnaissance mission to the scene of the grammatical crime, where one team member will take a picture of the offending work. Then, typically while sharing of beer (so much for “write drunk, edit sober”), they gather to debate the graffiti’s many grammatical failings: Where should the comma go? Is this letter supposed to be an “O” or an “A”? This can take a while — the team once found 13 mistakes in a simple, two-line message.

The final edit always happens after dark. Clad in hoodies and ski masks, they sneak out in pairs — one to act as lookout, one editor — to complete their operation. Because the three agentes are grammar nerds and not actual secret agents, these missions don’t always go according to plan.

“The first time we went out and corrected something, I was on lookout and I was supposed to whistle if I saw something,” Punto Final recalled in a phone interview with The Washington Post. “But the very first car that went by was a cop car, and all of a sudden I couldn’t whistle, so I yelled ‘caw caw’ instead.”

Outed by Punto Final’s unconvincing bird call, he and his partner wound up fleeing the scene, leaving behind their stencils and spray paint.

Their technique has improved in the past few months, though. Now Punto Final estimates that he can finish a job in about two minutes, including 30 seconds to spray on the stencil of his group’s name: Acción Ortográfica Quito. They could probably shave off another minute if the city’s graffiti artists would stop making so many mistakes.

In case it wasn’t obvious, Punto Final gets worked up about good grammar. Really worked up. He sees his graffiti-correcting as a “moral obligation,” he said.

“Punctuation matters, commas matter, accents matter,” he said. “This is one of those things that it’s not about, ‘Oh everyone has their own truths and nobody is the owner of the truth and we have to be tolerant.’ Sometimes people are just wrong.”

Within minutes, Punto Final starts to wax philosophical about the whole endeavor: “If you think about it, if you think about graffiti that is an act of vandalism, it’s anarchic in a way,” he said. “And if someone starts correcting the vandalism through vandalism, it’s kind of ironic. You would be putting some kind of order in something that has no rules.”

But the intention of Acción Ortográfica Quito isn’t to act as a schoolmaster-y slap on the wrist or a high-concept commentary on the plight of basic literacy in the age of texts and Twitter — or at least, it’s not mainly those things.

“Mostly it’s something to entertain the city, to make it less serious,” he said.

The group’s name was chosen to mock another guerrilla graffiti art movement, Acción Poeta, which has been painting poetry on city walls across Latin America since it began 20 years ago in Monterrey, Mexico. Punto Final admires Acción Poeta, but he says they take themselves a bit seriously — and could occasionally benefit from a good copy editor. He’s already corrected one of their Quito poems (below), and he’s more than willing to offer up his services again. 

Correction: An earlier version of this article misspelled Punto Final’s alias. It is Punto Final, not Ponto Final.