“The kind of arch it is, if you take a big chunk out at 2 o’clock, it will fall down,” Murphy says, not quite coming up with the word “catenary.” “The plating was ripped off during a Voton attack. The Arch miraculously didn’t come down. It held together for a reason that is part of the show’s mythology, but we won’t find out why the Arch didn’t fall until much later.”

Rafe McCawley (Graham Greene), “the richest guy in town, is a St. Louis native, and he wanted to repair the Arch,” Murphy explains. “The inner skeleton was beginning to show, and he worried that it was going to fall.”

McCawley’s own engineers tried everything they knew, “but it didn’t work, and the Arch was going to come down like the Leaning Tower of Pisa,” Murphy says, enjoying his story. The mayor — Amanda Rosewater, played by Julie Benz — “said we have to go to the Votons for help if we’re going to save the Arch.”

The result is “one of the first examples of aliens and humans coming together, all for the sake of the Arch,” Murphy says.

And they all lived happily ever after? Not quite. The situation is more complicated than a few patches.