A 5-year-old boy grabbed the attention of San Francisco and then the White House as thousands of residents joined an elaborate effort to help the tiny leukemia patient play crime-fighting "Batkid" for a day.
With the help of the Make-A-Wish Foundation, Miles Scott was rescuing a woman, capturing the evil Riddler during a bank robbery and stopping a kidnapping by another villain, the Penguin.
A grateful mayor Ed Lee planned to give the boy a key to the city after the "crooks" were caught.
Miles thought he was in San Francisco only to get a Batman costume so he could dress like his favourite superhero.
Instead, he saw a broadcast in the morning with Police Chief Greg Suhr calling for his help.
"This has turned into a full blown phenomenon," Suhr said.
The San Francisco Chronicle planned to distribute special-edition newspapers with the headline, "Batkid Saves City".
Miles took off from Union Square on Friday morning (local time) in one of two Batmobiles black Lamborghinis with Batman decals. Police escorted the cars and closed off roads as the boy, accompanied by an adult Batman impersonator, sped away to save a woman in distress on a cable car track.
The White House sent out a Tweet encouraging Batkid to "Go get 'em!"
The bashful Miles looked overwhelmed by the attention.
A crowd roared as Batman and Batkid rescued the woman, disabled a plastic replica bomb and moved on to their next adventure. The crowds grew after each one, chanting, "Bat kid, bat kid". Spectators climbed trees and clambered up lampposts.
The boy's father, Nick Scott, thanked the estimated 7,000 people who were making his son's wish come true.
Miles was diagnosed with leukemia when he was 18-months-old, ended treatments in June and is in remission.
Make-A-Wish has fulfilled similar wishes across the US One child became Batman's sidekick, Robin; and another was a secret agent, said Jen Wilson, a spokeswoman for the local organisation.
"When you have an illness, it's very important to know you have a support system," said one onlooker, Gina Futrell, 51, who has multiple sclerosis. "I have an extremely strong support system, and I hope he does too. He's such a little hero."