Ireland's Food Standards Authority tests beef products from major supermarkets and finds one sample is roughly one third horse DNA, a discovery that triggers a scandal spreading across 26 European countries. CEO Alan Reilly describes double- and triple-checking the lab results before going public. The film follows animal rights activist Stephen Philpott, who has spent years running surveillance on gangs smuggling thousands of unwanted Irish horses across the border for illegal slaughter. A whistleblower, filmed in fear for his life, admits forging horse passports that were never checked and drugging animals too weak to walk so they would survive the trip to the abattoir: "if you could get it up the ramp it would be on." The film traces the supply back to Ireland's construction boom, when builders bought horses and joined racing syndicates, then abandoned them in fields and roadsides once the housing bubble collapsed. Supermarkets, suppliers, and regulators each decline to take responsibility as the trail runs cold.