Crossover:
Stargate SG-1

Timeline:
Harry Potter: starting after the Battle of the D.o.M (June 1996)
Stargate: just before Stargate, the Movie (July 1996)
Stargate SG-1 (aired) begins July 1997.

While the Stargate movie was release in October 1994 the first episode of Stargate SG-1, Children of the Gods, establishes that the mission took place ONE year before the show.

Alterations to Canon:
None for Harry Potter, apart from fanon tropes.
Some for Stargate, specifically something different beneath Glastonbury Tor.

Pairings:
Harry/Hermione/Luna/Tonks

Story:
Following the Battle of the Department of Mysteries and Dumbledore's brilliant idea to choose that moment to reveal the prophecy, Harry finishes trashing Dumbledore's office and storms off. At first he is headed back to Gryffindor Tower but then he decides that he doesn't want dozens of people staring at him or asking questions. He considers going to the Hospital Wing to visit the others but finds himself suddenly lacking his Gryffindor courage. He can't decide who he is more angry with, Dumbledore for his shite or himself for leading his friends into life-threatening (or ending, in Sirius' case) danger.

Realising that the Room of Requirements is only a short walk away he sets out for it, intending to use it as a place just to take refuge. However when he's standing outside of it, wondering what to ask the Room for, he is struck by the inspiration that maybe he should ask it for help with his Voldemort problem. He paces back and forth, thinking of his need: 'The power to fulfil the prophecy, the power to defeat the Dark Lord, the power to save my friends'. The door appears and inside is a single small classroom desk and a bookcase, the later with only a couple dozen books in it and the former with just one resting on top.

The books in the bookcase appear to be Combat related, about how to use magic for warfare, but it is the one singled out on the table which seems to be the most mysterious. It's old, so old that the dialect of English is hard to decipher, even for a wizard with five years of experience in Hogwarts' library. The front cover has what looks like a rune of some sort and inside he finds the name 'Godric Gryffindor'. The journal details his life before he helped build Hogwarts, speaks of his quest to restore Albion to the glory of 'the Great Age' and his pilgrimage to 'Glastingberi'. There he met 'the Guardian of the Tor' and was enlightened before leaving with a gift, the 'machine of magic', now inspired to rebuild Albion to glory not through strength of might but by the power of education, his mission to build a school of magic.

Over the next two days Harry works hard at trying to decipher the rest of the book, spending hours in the library where Luna approaches him and helps him to find books that might help. Leaving the Hogwarts Express, Harry passes a letter to Hermione and leaves before she can talk to him or ask him what is going on. In it he apologises to her for not listening to her warnings and for what happened to her, that she should stay away from him and be safe... but despite having no right to ask for her help he is going to, he has found something and believes it may be a way to win the war, if she would be willing to meet him at Glastonbury in one week's time.

At Privet Drive Harry packs a backpack, makes more notes, contingency plans and finally visits the local train station to get a schedule. A week after arriving in Surrey he slips on his invisibility cloak and boards a train to Bristol and then hops a bus to Glastonbury. Arriving in town he is shocked to find Luna ooh-ing and aah-ing over the contents of a bakery's display window, she thought she'd do some research during the summer and give it to Harry when they got back to school. With Luna and a few baked treats in hand he heads for the town library to meet with Hermione who immediately glomps him, a hundred-words a minute pouring out about how she'll always be his friend, no matter what. They sit down at a table outside a cafe, order some tea and are about to eat the pastries when a fourth person strolls up and joins them.

Tonks smiles and winks at an unimpressed Harry, who else is coming, Mad-Eye, Snape? No, just her. How did she find him? Oh Harry, you should have worn your invisibility cloak to the train station. He did, he protests. Not on the day he grabbed the schedule, kind of made it obvious he was planning a trip, didn't it? Tonks tagged his shoes with a tracking charm pre-emptively and just followed that. So she's here to drag him back to Privet Drive? No, not unless he's planning to flee the country or something. She just figured he was sneaking off to go visit a girlfriend... not two! Hermione protests, blushing, that she isn't Harry's girlfriend and Luna asks Harry if she is his girlfriend, Harry stammering out that he doesn't think so (Oh... poo, says Luna). Well then, Tonks says, grin on her face, you must be single then Harry, so where are you taking me for lunch? Hermione scowls. Something wrong Hermione?

Harry produces the journal for Hermione and Tonks' consideration, he explains how he came by it and that he hopes it will help them win the war. Tonks frowns, she's not denying the part that they've played already, but they shouldn't be looking for trouble or danger, it's not their job to fight, not yet. Harry rather bluntly informs them all that it actually is, that there's a reason the Order was guarding the Department of Mysteries all year, that there's a prophecy... and 'It's me or him, one of us will kill the other, that's how it ends'.

A horrified Hermione is out of her chair glomping him again while a hesitant Luna places her own hand on his arm, smiling at him but saying nothing. Tonks clears her throat after a moment and simply says: 'Well then... I'm in'. If what Harry's saying is true then there's no point in having him sit around ass-end Surrey all summer while she and the Order wait around in the hot sun watching him under stuffy invisibility cloaks.

They investigate the Tor for hours, not sure what they are looking for, when Luna discovers the same symbol from the front of the journal engraved into one of the menhirs. It's not a rune Hermione declares almost instantly, Tonks agrees. They search for almost an hour for more but find nothing. Luna has been re-reading the journal and says she's found something, the mark was something Godric had seen during this pilgrimage, he said it marked the start of the 'Path of Moros'. When she speaks the words all four of them are bathed in a bright white light and suddenly find themselves standing in a dark place which lights up with burning torches.

The cavern, for they are definitely underground, is filled with empty, cobweb covered chests. Someone else robbed this place a long time ago, Tonks suggests. No, Hermione replies, it doesn't make sense. If you were going to empty a cave of a horde of treasure, you'd carry it out in the trunks it was being kept in... so what's with all the trunks? They continue to search and Harry finds it again, the Path of Moros symbol. He touches it and there's another flash and sudden a stone sits in the middle of the room with a sword stuck in it.

Excalibur? Tonks wonders in awe. Depending on which details of the various Cycles are true then it probably isn't, Hermione replies. There are at least two different accounts of swords drawn from stones. Ladies first? Harry offers. When he pulls the sword forth a plate-clad knight appears and lifts its sword, 'Prove your worth'. Harry refuses to do likewise, he didn't come to fight. What does he seek? Hermione goes to speak but Harry stops her, this is a test, he reasons. He seeks knowledge, no, he seeks to learn. The knight says his purpose is to guard the wisdom of Merlin, why should it permit Harry to have it? Harry asks if he deserves it. The knight stops and lowers his weapon. Because he seeks to learn, not to merely take knowledge, the knight judges him worthy.

There is another flash of light and suddenly the cavern, unchanged in size or dimensions, is filled, not with trash and detritus, but instead something akin to a muggle scientific laboratory, except perhaps even more advanced. Ranks of machines, banks of consoles, strange crystalline apparatuses and much more. The knight himself appears as well, a gruff-looking, ruggedly built man of maybe fifty years of age and introduces himself: Galahad. Yes, the knight of the round table. This place they stand in is Seige Perilous, Merlin's redoubt, and Galahad's station. He slumbers here for fifty years at a time, emerging for a month each time to maintain the lab and be updated on knowledge about the world, gathered by monitoring sensors in the intervening time - and how he can speak naturally in their language.

Merlin or Myrddin or Moros, was an Alteran, one of the precursors, the builders of Atlantis, who sailed the starts, settled and terraformed earth, seeded the galaxy with life. This lab was where he worked to prepare a defence against an enemy which seeks domination over all life, he disappeared centuries ago but left his ultimate duty to Galahad should just such a thing happen. He doesn't tell them any more than that but relates that he himself met with Godric and convinced him to give up his quest for power in order to seek a more enlightened path. Much knowledge could be made available to the four of them as well.

The Pairing:
Should gradually come into play as the four of them make preparations for taking down the Death Eaters, stripping the Supremacist faction of their power and finally fighting Voldemort.

The Restoration:
- Using the Alteran Database Interface (a.k.a the 'Headgrabber') the four of them receive piecemeal downloads of the Alteran knowledge base. Biology, Chemistry, Physics, Geology, Astronomy, History, Military Tactics and Strategy, Psychology, Social Science and much, much more. Revision sessions with Galahad allow them to integrate the data into their understanding, letting them use it, not just know it.

- Relating his experiences with Voldemort's wraith, the Dairy, his scar and the resurrection he witnessed, Harry gives Galahad the suspicion that perhaps the Dark Lord's survival may be related, in some demented and awful way, to ascension and later, before Harry and Company's return to Hogwarts, he presents them with something he worked on in response - a sword, in-laid with an energy pattern, a prototype of a weapon created by Merlin to be wielded against the 'Great Enemy' (the Ori) and designed to disrupt the energy-wavefunction of ascended beings. It's not an area-effect weapon, so to speak, but must be used to strike directly. With this weapon Harry will later sever Tom's connection to his horcruxes, destroying each anchor with each blow of the sword to Tom's homunculous until dispatching Tom for good.

- Being granted a breadth of knowledge that only some of the most qualified scientists and experts on the planet might possess, combined with a firm grounding in the scientific method, critical thinking and sceptical inquiry, dramatically changes the mindsets and methods of the quartet. They don't change in personality (much) but the way though go about making decisions and pursuing their goals does very much. Hermione begins re-inventing Arithmancy based on the advanced Pure Mathematics known to the Alterans, disproving rules and theories along the way, solving problems that have withstood scrutiny for centuries, paving the way to a methodical and reproducible model of magic. Luna tackles uniting potions, medicine, biology and physiology. Tonks compromises security on the Ministry with science, her morphing and social engineering/psychology and begins compiling very detailed dossiers on the Death Eaters and their assets. Harry begins experimenting with creating new spell effects based on harnessing physics and energies known to the Alterans and experiments with enchanting objects with this powers.

- Following the defeat of Voldemort the quartet decide to set about setting up a good foundation for a safer, saner magical society and then turn their gaze to the stars, looking to rebuild the Alteran society.

Additional Notes:
- The quartet are at Seige Perilous when the sensors detect the first activation of the Stargate in over 50 years. They fabricate a probe and send it to investigate and discover its location. Discovering that it's fairly inaccessible under American military control they obtain and secure the Antarctic gate for their own use. (This pre-empts several Stargate episodes).

- They've just finished off Voldemort and are solidifying their hold over the Ministry when Apophis' ships are destroyed in orbit. This hastens their efforts to get into space and establish defences.

- They establish a foundation/corporation recruiting muggleborns, squibs and halfbloods, mostly those living in the muggle world, for their ATA gene (almost 70% of UK magicals possess it) allowing them to receive some direct-knowledge download educations. ATA gene detection devices are set up in public spaces, triangulating candidates who are identified by facial-recognition software tapping into public CCTV networks, then algorithms compile dossiers on them, assessing them for possible recruitment. Purchasing manufacturers and laboratories allowing them to introduce improvements in materials, chemicals, engineering, technology, medicine, scientific instrumentation. Establishing charities to disseminate clean water technology, cheap medications, educational tools, sanitation and hygiene and more to the developing world.

- 'Fair Game', Season 3, results in the Asgard bringing the New Alteran faction to the negotiating table as part of the Asgard Protected Planets treaty. It creates a fair bit of tension, the American intelligence services having had their suspicions about the source of Alteran Foundation's technology, the untraceable nature of some of their activities, disappearance of scientists and experts recruited by them, but refusing to partner with any nations' armed forces. The New Alterans have needed to clean up after aborted and messy attempts by the NID and several other groups trying to infiltrate or sabotage their work. The end result of the negotiations doesn't bode well for a close working relationship or combining their efforts either. The treaty nominally bans Earth from advancing technologically but the Alterans' entire objective is to restore their technological society and it's derived from a knowledge base they already possess and merely are working to utilise - a knowledge base they are unwilling to turn over to the US Air Force to be used in off-world activities that they have no control over.

- Orbital infrastructure, concealed from detection (not unlike Asgard ships), moon facilities for heavy automated mining of metals and minerals as well as hangars for constructing basic recon scout ships, asteroid mining/towing vessels, scientific platforms (only the scouts possessing hyperspace capability at first). Asteroids placed into Earth's Lagrange points and turned into habitats and further engineering/manufacturing facilities. Training programs to create the necessary supply of skilled personnel for their objectives.

- That Jack O'Neill is an ATA carrier and survived an uncalibrated, unindexed download of Alteran knowledge impresses Harry and Company greatly. Jack finds himself in the amusing position of being the nominal 'contact of choice' for them as well, not just the Asgard. Samantha Carter is occasionally offered the opportunity to work on joint projects with Alteran Foundation scientists. Daniel is welcome to study non-restricted Alteran history and culture.

- Complications of the US Government discovering the connection between the New Alterans and the native magical population of Earth.

- Anubis' attempt to drop an asteroid on Earth, circumventing the Protected Planets Treaty, spurs Stargate Command and the New Alterans into working together to face the threat of the mad Goa'uld. Subsequently the Air Force diverts their resources from F-302s and building the X-303 to building up the New Altera Sol Defence Force.

- Certain reasonable obstructions to Harry and Company simply mass producing technology from the peak of Alteran supremacy. Fabrication of complicated technology larger than that which can be assembled with the Database Interface (like when Merlin fabricates the components for the San Graal) is much more difficult. Creating the subspace-time dimensional pockets that hold the vacuum energy inside ZPMs requires a facility constructed in orbit around a rotating black-hole to use its influence to fold space, a process that even space-expansion magical techniques can't adequately reproduce - meaning years of infrastructure needs to be established first.