Very, very mild spoilers for "DC's Legends of Tomorrow" past this point.

"Time changes. People don't."

Rip Hunter (Arthur Darvill) tells one of the other members of his ragtag super-team the above late in the second episode of The CW's upcoming, excellent team-up show, "DC's Legends of Tomorrow" (which premieres on January 21). And pretty much everything you need to know about the series is encapsulated in those four words (or at least based on the two-part pilot episode that was sent out as screeners by the network).

Basically, between all the explosions, superheroics, sci-fi jargon and laugh-out-loud funny quips, there's a much darker, sadder idea skimming below the surface... That no matter how hard we try, our lives don't mean much; and no matter how much we try to change ourselves to try and make our lives matter, they never, ever will.

Basically, "Legends of Tomorrow" is the "Avengers" of sadness.

But also there's explosions! And romance! And all the soap opera meets super-powered trappings we've come to expect from The CW's two other DC Comics based shows (that would be "Arrow" and "The Flash"), from which "Legends" spins off. And before you get concerned that you haven't watched three seasons of "Arrow" and a season and a half of "The Flash" and will be instantly lost, "Legends" gets you caught up to speed in record time.

The gist of it? Rip Hunter is a time traveler looking to stop the immortal villain Vandal Savage (Casper Crump) from taking over the world. To do that, he assembles a team: the arrogant Dr. Martin Stein (Victor Garber), who is also one half of the powerful hero Firestorm, along with former football star Jefferson Jackson (Franz Drameh); the earnest, size-changing, super-suit wearing Atom, former billionaire Ray Palmer (Brandon Routh); a conflicted, formerly dead assassin named Sara Lance who -- mild spoiler -- now goes by White Canary (Caity Lotz); two thieves with a cold gun and a heat gun respectively (Wentworth Miller and Dominic Purcell); and Hawkman and Hawkgirl, arguably the most confusing heroes in the DC staple (Falk Hentschel and Ciara Renée).

It's with the latter that it becomes clear how good the "Legends" team is with distilling information into bite-sized nuggets. The winged heroes are actually reincarnated lovers, destined to die at the hand of Vandal Savage once a generation. In their latest incarnation, Hawkgirl -- who now goes by Kendra -- hasn't gotten her memories back, so isn't aware of how much she loves Hawkman the way he loves her.

That the show gets all of this information out without causing the viewer a massive, continuity filled headache is a testament to how well the writers get the plot and character across through action, versus everyone standing around and just talking to each other (though there is some of that, too).

Those viewers not looking to delve further below the surface of "Legends" will be treated to an infectiously enjoyable superhero epic, as the team gets assembled to take down Savage -- and of course, runs into multiple fights along the way. Those fights, by the way, are pretty much as monumental as you'd want from a show packed with eight superheroes. Cribbing liberally (and often) from that tracking shot at the end of the first "Avengers" movie, "Legends" looks and feels like The CW spent the budget of an entire season of "Hart of Dixie" at least twice every episode.

"Arrow" has some of the best stunt work on television; "The Flash" has at least one stand-out special effects moment every episode. "Legends" drops both of those elements in several times in each of the first few hours -- and the result is giddy, jaw-dropping fun on par with anything appearing in movie theaters.

Not only that, but the character interactions are a delight. The show spends a good portion of the first two episodes mixing up the rather large team into different iterations (for good reasons given their varying missions, so it never feels forced) -- mostly just to see what happens when, say, the straight arrow Palmer ends up with two career criminals; or Stein is forced to work with two team-members he (initially) considers below his intelligence.

In fact, almost all of the characters work splendidly. Lotz gets some of the best action work on the show, and it's great to see her flex her muscles after being benched for the last season of "Arrow" (though technically her character was dead, so whatever). Garber is, of course, reliably hilarious; Purcell, who never really clicked on "The Flash," finally comes into his own, while Miller continues to be a wickedly enjoyable anti-hero; Renée and Hentschel provide the needed romantic tension, and some great emotional work as the show continues; and Drameh and Routh both get stand-out comedy moments, but prove to be the real heart of the team.

Even Crump, who's main threat seemed to be that he had a lot of knives in his coat when his Vandal Savage first appeared earlier this season on "Flash" and "Arrow" gets to flex his villainous muscles, and emerges as a real threat that could take on eight perfectly capable superheroes at one time.

It's not too much of a surprise that they all work so well, as each of those characters have previously been established on "Arrow," "The Flash" or both. Some have histories together, some don't, but they all continue their own, ongoing conflicts and stories on "Legends." It's a smart move that allows fans to follow their favorites in their next adventure; while cutting out a lot of the lack of confidence in secondary characters that can put newer viewers off trying shows after the pilot.

The one element that's not quite working, yet, is Darvill's Hunter. Darvill was reliably goofy as companion Rory on "Doctor Who;" and not that there's anything against an actor stretching their muscles, but Hunter is mainly just an angry twit in the first two episodes. He's The Doctor (from "Doctor Who"), but without any of that character's humor, or wonder at the mysteries of time travel. There's certainly an inkling of where they're going to go with Hunter by the end of episode two: a lot of his arc seems to be about embracing his destiny as a hero and leader -- particularly when he's assembled a team of characters that includes several other leaders who might be more capable than him. But for now, unlike the rest of the characters who the writers have already had time to figure out, Hunter is a work in progress.

That's kind of the theme though, right? And it's that deeper level that elevates "Legends" beyond your run-of-the-mill syndicated, afternoon sci-fi show. Like the plot bits, which could easily weigh down the show but never do, the emotional undercurrent of loss and failure permeates every move the characters make, without dragging us down into the land of mopey, crying superheroes who never smile. Sure, there are points when the characters say what they're feeling, and lament their place in history -- but right after that, we're into a comedy set-piece, or an action sequence.

But that thought is always there, that no matter how many people they save, or adventures they'll go on, everybody on Earth will eventually be forgotten by the inevitable march of time. Routh exemplifies this best, as he explains in the first episode: he was a billionaire, a superhero, and even saved his city... But when he "died" last season on "Arrow," life just went on.

Same with Garber's Stein, who is faced with the stark reality of how little he's changed in episode two. And of course Hawkman and Hawkgirl, who have lived thousands of years, through hundreds of lifetimes, replaying the same deadly dance time and again.

On the opposite end of the spectrum is Savage, who's whole goal is to make the world remember him by conquering it. He's moved through history as a ghost, subtly influencing events from behind the curtain... But by the time "Legends" opens, he's ready to openly change the world so no one will ever forget the name Vandal Savage.

This is where "Legends" gets to the heart of what works so well with the superhero genre, when it's working as well as it does here. Who hasn't felt the twinge of sadness when they realize that ultimately, they'll never make the impact on the world they think they will... And even when you're, say, Albert Einstein, or Adolph Hitler, there will eventually be another Einstein or (sadly) Hilter down the road who will replace you? And then you realize there was someone before you, too. Fun fact, for centuries before Hitler, the Egyptian Pharaoh was the sub-in for the now popular phrase "worse than Hitler." Even with centuries of use, Pharaoh's been forgotten with time, so will Hitler, and so will you.

That's what superhero shows and movies at their best strive to do, and "Legends" captures expertly: getting to the heart of something we all feel, and distilling it through the frame of superhero metaphor. Like we mentioned earlier, this sadness, this sense of abject failure doesn't permeate the proceedings because it's usually too concerned with making you giggle with glee at the ridiculous heroics happening on screen. But the idea is always there, and pops up when you least expect it to.

Ultimately, even if all of us reading (and writing) this review won't reach the heights of an Einstein, or the depths of a Hitler, "Legends" comes around to the other idea that makes the superhero genre the predominant form of entertainment in 2015/2016: that it doesn't matter whether you're remembered, or thanked; become a legend, or disappear into the pages of history. The only thing that matters is that you try to do the right thing. Even if people can't change, you still have to keep trying, because that's what makes us human -- and them superhuman.

But along the way, the "Legends" -- and us, as the viewers -- will continue to struggle with the nagging doubt that maybe that doesn't matter in the long run, either... Between all the ridiculous fight scenes, of course.