Here is everything you might want to know about this character, and here is a simplified version: Ystina is a young girl who disguised herself as a boy to become Sir Justin, a member of King Arthur’s Knights of the Round Table circa 8,000 BC (Camelot, as Morrison establishes, is a recurring archetype, but Ystina’s Camelot is the first). The evil queen Gloriana Tenebrae destroys Camelot, and in escaping, Ystina accidentally time-travels to the present day with the sword Excalibur, and her winged horse Vanguard. After Gloriana is defeated, Ystina is enrolled in the “H.S. Johnson School for Heroes” by Ali-Ka-Zoom, a homeless mystic, and on one of the last pages of Seven Soldiers #1, he explicitly sets up of the potential ongoing series:
“…you’ll have fun here. You need to learn some more about the 21st century and how it works before you go swinging that sword all indiscriminate. Weekends you and your horse can fight the good fight all you want. I can’t stop you, only give you advice … even if you do decide to start up your own round table with all the new friends you’re gonna make…”
Morrison’s made it very simple, right? Girl knight with mentor figure, lost in time, can’t go home, enrolled at some kind of hero academy. So … what happens?
Some of it is her “weekends.” She’ll fight random monsters, of course, but we’ll only see enough of that to get the sense that it happens all the time (just like when Spider-Man spends a page stopping a mugger; it’s not the main thrust of the story, just an excuse for a quick action scene). Most of the on-panel time will be spent on more interesting and bizarre adventures, chief among them the Quest of the Three King Arthurs. We are told by Gloriana that after the original Arthur from Ystina’s Camelot, “There were of course several Arthurs; a pagan general in Roman Britian, a medieval Christian mystic…” Gloriana knew about, but did not mention, the King Arthur of the 109th Century AD. There is, however, only ever the one Merlin, and it is he that brings the three of them together to enlist the help of the last surviving knight of the Primal Round Table in the search for a treasure that loses itself in time. Together they embark on a series of journeys that culminate in 12th century England, where they also discover the terrible origin of the Sheriff of Nottingham’s Clockwork Man technology, and the tragic tale of how Robin Hood really died.
There is, of course, a King Arthur in the present day as well, but just how Aquaman fits into the legacy is a mystery he and Ystina will have to work together to solve.
But it’s not all epic quests through time, because there’s still the school five days a week. Note that it’s not a school for superheroes, but a school for heroes. School policy impels the faculty, which includes Arn “Iron” Munro among its members, to actively discourage costumed, superpowered heroism; but to understand why, you must also understand why this H.S. Johnson formed the school in the first place, and none of the students are permitted to know.
The star student at the school is Ranger St. Clair, who you might imagine to be exactly like Doc Savage, except three months shy of legal driving age. Billy Beezer, a former member of Mister Melmoth’s child gang, enrolled at the school after escaping the forced labor camp on Mars in Morrison’s Frankenstein! series; he tries to become a hero and leave his former days of hedonism and petty crimes behind, but when some of his former gang members turn up at the school, he’ll have to deal with temptation.
Ystina is placed in one of the school’s special classes (“Well, they’re all special classes, aren’t they?” the school administrator says). It’s called the TODAY program, and it’s designed to help other temporally displaced youth adjust to life in the 21st century. From the past there’s Juan-Carlos Canyon from the Old West brought to the present by aliens, and Victor Victorian, whose interest in séances led him into the mysterious limbo known as the Ghost Realm in 1897, only to re-emerge six months ago with the ability to commune with and control spirits. Brash and callow Axel Strange claims he’s Adam Strange’s grandson but can’t prove it, and isn’t saying why he finds himself in our time, but he’d love to supplant Ranger St. Clair as the school’s top hotshot. You might assume Shakespeare Kid is from about 1600 AD, but you’d be wrong – “Shakesy” is a member of the Legion of Substitute Heroes in the 31st century and hopes to become a full Legionnaire after learning some valuable lessons at H.S. Johnson, but has developed a curious interest in Axel in the meantime...
Ystina is extremely serious and grave, and Ali-Ka-Zoom is there not only for advice, but also to help her lighten up. Of course, her new friends in the TODAY program might help as well; are they to become the first of the prophesied Queen Ystina’s new Round Table, or are they just a bunch of weirdos who can’t work a toaster? And when Iron Munro goes missing tries to recruit the Leviathan entity (which is, as you’ll recall from Morrison's Klarion, made up of 125 lost children underneath the subways of New York City), will the TODAY class save the day, or will they need Ranger St. Clair’s Young All-Stars to rescue them as well?
Next: A witch-boy and his monster.