Spoilers from last night’s Supergirl ahead.

Comic nerds were in for a treat during last night’s Supergirl, the aptly-titled “Human for a Day.” Hank Henshaw, commander of the DEO, was presumed to be exactly who he was in DC lore, a bad guy who becomes Cyborg Superman following the infamous “Death of Superman” story from 1992.

Well SURPRISE! Turns out, Henshaw as we knew him was the Martian Manhunter himself, J’onn J’onzz in disguise. A legit Justice Leaguer is in Supergirl!

With DEO holding Jemm (coincidentally portrayed by Constantine actor Charles Halford) in custody, an earthquake strikes and powers down HQ, unleashing Jemm and his mind control loose in the base. When Alex’s distrust of Henshaw reaches its apex, she traps the DEO director and hopes to take down Jemm on her own. She doesn’t, and Henshaw comes to her rescue. “I told you, I’m not the enemy.”

He certainly isn’t. In a stunning reveal, “Henshaw” explains Jeremiah’s (Alex’s father, Kara’s foster) brief time with the DEO, telling Alex he was a good, compassionate man while the real Hank Henshaw was kind of a dick. Henshaw and Danvers went to Peru to find J’onn, a refugee after his Martian civilization had been wiped out. After both men were killed, J’onn took on Henshaw’s identity to reform the DEO and look after Alex as a promise to Jeremiah.

Yeah there are holes in that story, but who cares right now? The freaking Martian Manhunter is now in the Arrowverse(?)! This isn’t the first time he’s been on live-action network TV (actor Phil Morris played the character on Smallville), but he’s pretty much in his entire glory on Supergirl.

The inclusion of the character actually came as an impromptu thing. Series co-creator Andrew Kreisberg told Collider:

“We were just so in love with David, and so happy to have him on the show and just loved everything he was doing with Hank.” He added, “I don’t remember which one of us said it, but it was like, ‘Oh, if we were ever going to make a Martian Manhunter show or something, David would’ve been the perfect Martian Manhunter.’ And it was Geoff who said, ‘Well, why can’t he be?’”

Once again talking with Collider, Supergirl costume designer Kiersten Ronning explained the design of the Martian Manhunter suit, which takes clear inspiration from his comic book look with typical modern aesthetics:

“[We wanted to make sure] the color palette complimented Supergirl,” said Ronning. “The way it’s built, it moves well. It’s not heavy. Having two superheroes in capes can be difficult, so we built his more off the shoulders, where hers kind of comes over, trying to keep it masculine.”

OK, but who is the Martian Manhunter?

The character has had a ton of varying origins due to reboots and retcons, but it was originally this: Martian Manhunter is J’onn J’onzz, an alien from Mars (where else?) pulled to Earth by an experimental teleportation by Dr. Saul Erdel. After telling Dr. Erdel where he’s from, Erdel dies of shock and J’onn is left stranded on Earth until the day Martian technology catches up to bring him back. Until then, J’onn adopts the identity John Jones and becomes a detective in Middletown, U.S.

Along with typical superhero abilities (flight, super strength, regenerative healing), he’s also armed with psychic powers like telekinesis, telepathy, and a genius-level intellect, making him not unlike a warrior monk. Most importantly, J’onn is capable of shapeshifting, which allows him to hide among humans, most certainly as Henshaw in Supergirl. At some point J’onn was made as the last of his kind, which allowed him to find kinship with another alien who has lost his people, Superman.

Speaking of Superman, Martian Manhunter is one of the original founding members of the Justice League.

Alex Ross' painting of the original Justice League, Martian Manhunter among them.

What does Martian Manhunter’s presence in ‘Supergirl’ mean?

Good question. While J’onn could help Kara establish a kind of Justice League — even though Superman is well into this career, it seems he hasn’t met any Manly Bats or Wonderful Women — it’s too early in Supergirl for Kara to team with others when she can barely handle the responsibility on her own. Plus Legends of Tomorrow is in the near future and Berlanti Productions might be up to their ears in juggling all these costumed freaks into a singular coherent TV story.

Before Supergirl can think about having a Justice League, it will need to answer one question: Is it a part of the Arrowverse or not? There are pros and cons for Supergirl to be a part of any established continuity, but if Kara wants to explore DC’s expansive history, she can’t ignore the one assembling (ha!) over at The CW.

Photos via DC Comics, CBS Networks