The first season of Marvel's Agents of S.H.I.E.L.D. was the definition of a slow burn. A long, early string of disjointed early episodes caused initial viewership to drop dramatically while the series struggled to find its footing. But then, late in its run, loyal watchers were finally rewarded for their patience with a strong finish that bodes well for season two.

What changed? As it turned out, Agents of S.H.I.E.L.D. was required to hold back several major plot developments in anticipation of the release of Captain America: The Winter Solider, and the show improved significantly when it was free to unleash those plot twists. But a second, equally important development was Agents of S.H.I.E.L.D.'s commitment to fleshing out its female characters. While almost no one was watching, the show quietly leaned in.

Who knew that Sheryl Sandberg was having a moment in the Marvel cinematic universe?

As a genre, superhero films have consistently come under fire for their consistent imbalance in male and female characters. Marvel Studios has responded to calls for superheroines by showcasing a bare minimum of female interaction and dropping occasional, noncommittal comments about Black Widow or Ms. Marvel movies.

But by any measure, the degree to which Agents of S.H.I.E.L.D. has embraced its female characters places the show in a league of its own. There are plenty of instances in which the show's main female agents — Skye, Melinda May, and Jemma Simmons — pass the so-called Bechdel test, defined as two named female characters having a conversation about something other than men. This includes an amazing episode that featured Momma May picking her up daughter in the Canadian wilderness, only to immediately scold her for not saying "thank you."

What's even more significant is the degree to which these women are portrayed as fully equal team members; though Iron Man 3's Pepper Potts and Thor: The Dark World's Jane Foster carried their films over Bechdel's finish line, you'd be hard pressed to find a fan who considers them to be anything other than secondary characters.

And with former team specialist Grant Ward having switched sides, Agents of S.H.I.E.L.D. is the first Marvel show or movie in which the majority of the heroes are women. The gender balance of the core team of agents also speaks volumes; the trio of females works alongside the boys without subjugation, and all three lean into their roles in ways that are textbook Sandberg.

Still not convinced? Allow me to match most of Lean In's major principles with examples from Agents of S.H.I.E.L.D.:

1. The Leadership Ambition Gap: What Would You Do If You Weren't Afraid?

May overcomes the trauma that earned her the nickname The Cavalry, transitioning from her self-imposed exile in a cubicle to piloting the Bus and going all-in for combat missions.

2. Sit at the Table

Simmons proudly claims ownership of her two doctoral degrees, and convinces her partner Fitz to join the team despite their lack of experience in the field.

3. Success and Likeability

While other agents of similar capabilities are treated with skepticism, May's level of badassery is admired and respected by everyone she comes across.

4. It's a Jungle Gym, Not a Ladder

To earn her spot, Skye hacks her way onto the team, completely disregarding S.H.I.E.L.D. protocols.

5. Are You My Mentor?

After coming clean about her real interest in S.H.I.E.L.D., Skye's relationship with Coulson deepens to one of mutual respect and appreciation.

6. Seek and Speak Your Truth

Following the collapse of S.H.I.E.L.D., Simmons openly, though respectfully, begins to question orders and advocate for her point of view.

7. Make Your Partner a Real Partner

Fitz and Simmons blend together like peanut butter and jelly (or scones and clotted cream).

8. The Myth of Doing It All

May's cold demeanor is often mistaken as a coping mechanism — but really, she's just mining her emotions and channeling her energy into some kickass hate-fu.

9. Let's Start Talking About It

Never one to shy away from stating the obvious, Skye correctly breaks down a dilemma in the field to the difference between girl parts and boy parts.

10. Working Together Toward Equality

Despite their initial reservations about each other, Skye and May come to admire each other over the course of the season.

While Agents of S.H.I.E.L.D. was always expected to introduce strong female characters — it is, after all, a Whedon family production — the extent to which the female leads have been developed is exactly the sort of expansion that studio executives have argued audiences wouldn't embrace. These claims were based on the notion that the traditionally male-dominated audiences who gravitate toward comic book stories would feel alienated by female protagonists.

But despite these assertions, Agents of S.H.I.E.L.D. actually performed outstandingly well in the ratings; the series ranked number one in the key male 18-to-34-year-old demographic for all 22 episodes this season, and was the top Tuesday show for men ages 18 to 49. Thanks to Skye, Simmons, and May, the argument for a boys-only superhero club is null and void.

Returning to Marvel's comic book roots, the recent naming of black hispanic Miles Morales as the new Spider-Man in the Ultimate universe, and the Muslim Kamala Khan as Ms. Marvel in the Earth-616 universe, suggest a brand that is moving towards diversity. And the successful approach championed by Agents of S.H.I.E.L.D. clearly hasn't gone unnoticed; ABC is set to premiere Agent Carter next fall, which will follow Captain America's Agent Peggy Carter as she builds the original S.H.I.E.L.D.

Couple this with Netflix's upcoming Jessica Jones show, and there's the distinct impression that Marvel might just be leaning in a little bit more — so much so that a major superheroine-led film is starting to look like a distinct possibility.