An LAPD detective named Mark Meachum gets pulled into a secret multi-agency task force after a Homeland Security officer is killed in public, in daylight, with no obvious motive. The investigation expands fast: what looks like a targeted killing turns into something with city-wide stakes and a ticking clock. Countdown plays its procedural cards straight, mixing the undercover thriller format with the classic race-against-time structure, set against Los Angeles as a city full of competing jurisdictions and buried loyalties.