The NASA shuttle containing Captain Marvel's Alpha Flight crew just disappeared before her, leaving no trace. She finds a mysterious station with the Alpha Flight insignia, but, upon going inside, she finds a slightly different Puck, Sasquatch, and Wendy. They also call their crew Zeta Flight. They soon find themselves under attack.
On a distant planet, Eve and Mim are searching for an artifact.
As you guessed, this is another mirror universe adventure. Funnily enough, Captain Marvel herself discovers that fact quickly without any major shock. Thus, is the life of a Marvel superhero, I suppose.
The Zeta Flight crew are slight inverses of their Alpha Flight counterparts. Puck is more timid, Sasquatch more talkative, and Wendy comfortable in action.
Zeta Flight also frequently reference some big score they've been planning, implying they are thieves and/or pirates. Their attacker, upon confronting them, also goes on about the score and implies that Captain Marvel will betray them.
Usually, in mirror universe stories, there is some sort of dramatic inversion of the characters we know and love. Otherwise, there is some sort of seemingly small yet somehow incredibly significant difference causing the major alterations to the universe. If there is one here, this issue does not reveal it. Zeta Flight is no polar opposite of Alpha Flight. In fact, they kind of resemble the modern Guardians of the Galaxy. They're swashbuckling space-thieves with trust issues.
(spoiler) The most significant difference is that Black Widow is alive, calling herself the Widower (clever, actually), and is the one attacking Zeta Flight. That heart-wrenching change will be somewhat undercut if Winter Soldier and Hawkeye find her in the upcoming Tales to Astonish miniseries (Bucky and Clint? you bet your ass I'm reviewing that). The reunion isn't too dramatic, as Widow spends most of her time declaring how much of a scumbag this version of Carol is while the Captain stands in shock.
Michele Bandini's art is really good in this issue. His work is usually solid, but it especially impressed in #126. The characters and details are well-defined, Captain Marvel and the Widower look awesome. The only thing that kept throwing me is that I couldn't tell if Sasquatch had a bald spot or blond highlights. That's not that big a deal, because either answer is funny. Erick Arciniega's color work is bright and dazzling, and the whole comic really shines aesthetically this time around.
Captain Marvel #126 is a decent read. The premise is interesting. Nothing especially exciting or dramatic happens. We get to see Carol shoot down some ships, but we get that one fairly often anyway. If you're a Captain Marvel fan like me, you will probably get something out of it. Otherwise, it'll probably just give you some mild entertainment. I do recommend it, but not strongly.