Justin Hartley's Tracker is easily CBS' biggest hit show - so why has it gone through three cast shakeups in just two seasons? When Tracker premiered in 2024, it introduced Us to survivalist Colter Shaw (Hartley) as he travels the country to help find missing people, track down information on criminal cases and more. The procedural quickly found success after being coined the most-watched broadcast premiere since NCIS' final episode with Mark Harmon in September 2021. Its average viewership was reported to be 18.2 million per episode, according to Paramount+ and broadcast measurements.