A massive cyber attack on computer networks around the world caused chaos on Friday.
A harmful computer program called WannaCry started taking over people's computer files and demanding money to let them back into their own systems.
But how was it stopped? Well, 22-year-old Marcus - a UK security researcher who has his own company - helped to limit the damage.
He's being called an "accidental hero" after registering a domain name to track the spread of the virus, which actually ended up stopping it from spreading!