When a Homeland Security officer (Milo Ventimiglia is starting to become the next Sean Bean!) is killed in the middle of a street in broad daylight, a task force is put together in the Countdown series premiere. It quickly gets into the action, while also teasing a potential storyline.
Caution: This post contains SPOILERS for Countdown season 1, episode 1.
The episode opens with the attack on Ventimiglia's Homeland Security character. He initially gets away, and just as it looks like he's safe, he's shot at close range. The bullet to the chest immediately kills him, and so the investigation into him and the shooting begins.

Nathan Blythe was investigating Homeland Security before this in Countdown
It's soon clear that there's a chance that this Homeland Security officer was in on something much bigger with the cartel that he was supposed to be investigating in Countdown. After all, he had money being transferred into his account. So, it means a trip to his workplace to talk to his colleagues.
One is quick to point out that there was a holiday cabin. That turns out to be false, and it's soon clear that this colleague was the one actually in on in. Mark Meachum (Jensen Ackles) and Amber Oliveras (Jessica Camacho) end up in the interrogation room, where they're constantly at each other's throats enough to confuse the suspect so that he actually talks.
It's great news for the wife and son of the fallen Homeland Security officer. However, the task force needs to get to the bottom of the entire situation.
One thing Meachum works out though is that Nathan Blythe (Eric Dane) put this task force together way too quickly, and it means he was investigating something before the shooting. Sure enough, he was, and he's put together a task force of different agencies because he doesn't know who he can trust.
Meachum has also worked out why Blythe chose the people he did. They are all people who don't work well with their agencies, and they won't be missed if something goes wrong.

There's a much bigger conspiracy at work in Countdown
As the trailer for Countdown told us, this case is much bigger than it initially seems. As the group take down the cartel and get justice for the fallen officer, Keyonte Bell (Elliot Knight) stops everyone from going to the shipping container. He knows exactly what he's looking at, and he rushes back to his car to pull out a radiation monitor.
Sure enough, there is radiation coming from that shipping container. Blythe points out that based on Bell's words, there's enough radiation in that shipping container to create a Chernobyl-sized event. Meachum may not have understood the technical terms, but he gets what that means for Los Angeles and everyone else.
I do have to wonder if we'll see Meachum risk his life around the radiation. During the Countdown series premiere, it's revealed that he's suffering from a brain tumor. The risk of surgery outweighs the benefits, so he just has to manage the pain, and his doctor wants him to take time off and enjoy the rest of his life. Meachum can't do that, and he's using his last months or years (however long he has, because it's not revealed) to do some good, spending the nine months before Countdown working undercover in a prison.
Meanwhile, Oliveras is viewed as a drug addict, but she's never been caught in a random drug screening. She has personal beef with Meachum, as it turns out that Meachum left his fiancee (her friend) at the altar. This is likely when he learned about the tumor, deciding not to leave his wife a widow so early, and we're sure to see that play out as the season continues.
For now, we've got a brief look at the various characters in Countdown, and it's clear there's a major conspiracy and nobody knows who to trust.
Countdown airs on Wednesdays on Prime Video.
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