Thor's not exactly the star of our show in this one. Don’t feel too bad. Dude has at least three other movies where he gets center stage. Plus: he's a Norse god, so he's got that going for him. Instead, we like to think of him as a utility infielder in this one: ready when called on to help the team out in subtle, but important, ways. Let's count those ways, shall we?
In the beginning of the film, Thor spends his time adding serious muscle to the Avengers' attacks. He flies, he pounds, he calls down lighting—you know the drill. But all that stuff is really just about what he can do, not so much about who he is. We don't really get to spend much quality time with him until after he gets mind-melted by Wanda Maximoff.
In the ensuing hallucination, Thor sees his Asgardian bud Heimdall, who informs Thor that Thor is leading everyone to hell. As you might imagine, this is the kind of message that can stick with a guy. Rather than regrouping with his fellow Avengers at Hawkeye's home in the country, Thor takes a quick side trip to London to learn more about what he saw in when he was under Wanda's spell. While there, he hops in "The Water of Sight." Think of it as a cosmic hot tub, only it looks like a giant, dirty puddle that hasn't been cleaned in centuries.
Even still, the Water does the trick. It shows Thor the truth about the Mind Stone (the gem hidden inside Loki's scepter and now planted firmly between the eyes of Ultron's android prototype), which motivates him to zip back to New York and bring the android to life with some well-placed lightning bolts. This is quite the executive decision, as Captain America and Iron Man were fighting about whether to do this just prior to Thor's arrival.
Our Norse god hasn't got time for fussin' around, though. Once his "vision" springs to life, Thor very helpfully relays what he saw in the Water of Sight, filling in both his teammates and us in the audience about how everything fits together:
Thor: I've had a vision: a whirlpool that sucks in all hope of life and at its center is that.
Bruce Banner: What, the gem?
Thor: It's the Mind Stone, one of the six Infinity Stones: the greatest power in the universe, unparalleled in its destructive capabilities.
Thanks to both Thor's detective work and decisive action, the Avengers add the new android (now called "Vision," appropriately enough) to the team, calling on this awesome new power to defeat Ultron once and for all.
In that way, really, without Thor, the Avengers wouldn’t have stood a chance. Sure he's loud, brash, and he uses too many "thee's" and "thou's," but the guy sure is handy. We'll give him that.