How people end up on Atlantis varies; some like talking about it and some don’t. Harry doesn’t mind telling anyone willing to listen: his path was a winding one, and it was all Niall’s fault.
An Air Corps training exercise went wrong because of some Ancient artifact previously undiscovered on the day Niall finally had clearance to show Harry around.
The artifact had been dormant for who knows how long until Harry bumped into the wall concealing it.
The fact that his ‘ATA gene’ was as strong as some American general meant he didn’t have a choice about being recruited into the Stargate Program.
Niall had a bit of the gene, too, so when he demanded to be allowed to stay with Harry, it
in fact allowed.
Atlantis always needed the gene, and even though it meant leaving Mum and Gemma, Harry wanted to go. Because Niall was the best mate in any world, he followed.
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