What if you could take up swords against the Lannister family on “Game of Thrones”? Or solve mysteries with the “NCIS” crew? Or pitch an ad campaign to Don Draper on “Mad Men”?

And then: What if you could watch the consequences of your actions on TV the next week?

That’s the premise behind “Defiance” (for the Xbox 360, PlayStation 3, PC, $59.99), a collaboration between the online game studio Trion Worlds and cable TV’s Syfy. By the time “Defiance” the TV show debuts Monday, “Defiance” the video game will have been out for a few weeks – enough time for players to make their own mark on this new universe.

Both the game and the TV drama are set in 2046, some 30 years after the Votan collective of alien species arrived in the skies over Earth. After a brutal war, the humans and aliens have settled into an uneasy peace, but alien technology that crashed to Earth has changed the landscape.

The “Defiance” game shows the effects of these “arkfalls” on California’s Bay Area, now a wasteland packed with bloodthirsty mutants, hostile cyborgs and overgrown, fire-spewing insects. Your character is an ark hunter who makes a living by scavenging from crash sites, and the search for a particular artifact brings you to the West Coast.

Soon after your arrival, the game’s sprawling map opens up, letting you choose your mission. You can race dune buggies around the wilderness, infiltrate raider strongholds and steal their loot or rescue farmers from “hellbug” infestations. Most missions can be handled solo, but if you stumble across a major arkfall you’re going to need help from other online players.

You’ll also discover “episode missions” that relate to the next week’s installment of the “Defiance” TV show. In the first such adventure, you meet military veteran Joshua Nolan and his partner, an alien named Irisa. They ask for your help retrieving a lost Votan doohickey, which turns out to be a significant plot device in the premiere of the Syfy drama.

But both sides of the “Defiance” team have collaborated on building an impressive world, and I’m eager to see where they go from week to week.

I was able to battle through the initial batch of episode missions in just a few hours, but there’s plenty more to do. As with any online shooter, you can engage in raucous death matches with your fellow humans. Or you can enroll in the Shadow War, in which huge teams of up to 64 players each battle for control of sites all over the map.

There’s also a wearying sameness to the bulk of the missions, which typically consist of racing to a location, killing a bunch of monsters and retrieving some object. The action is intense and challenging, often reminiscent of 2K Games’ fine “Borderlands.” But it’s missing that series’ twisted sense of humor, and I’m hoping Trion delivers more variety in future episodes. It’s a work in progress; for now, I give it two stars out of four.