As you can see in the third panel, DC hereby established a hard date: Supergirl's rocket arrived on Earth precisely on May 18, 1959.
Arguing from the typical logical fallacy Post hoc ergo propter hoc, Karen therefore blames Supergirl's rocket for the fire. In response, Supergirl narrates for her, and us the readers, the story of her origins.
Breathes there a man with soul so dead, that never to himself has said, "I know Supergirl's origin story"? If so, here's a refresher!
Isn't it handy that the domed Argo City was well-equipped with those handy-dandy Air and Food Machines?
However, the ground (not anything else, I guess) became deadly Kryptonite due to the explosion. So wasn't it also handy that Zor-El (brother of Superman's dad Jor-El) happened to have enough rolled-up lead sheet -- just kicking around in his lab -- to cover THE ENTIRE CITY!?@?
Then, after Kara has grown to young womanhood, meteors smash through the sheeting, exposing the inhabitants of Argo City once more to the deadly kryptonite rays.
Doesn't it strike you as odd that no mention is made of the fact that, before puncturing the lead ground covering, these meteors PUNCHED HOLES IN THE DOME? Well, a later tale sort of addresses this question. More on this, possibly, anon.
As you can see in this flashback, Supergirl's parents sent her to Earth after discovering that her cousin Superman was there serving as Earth's champion. We're supposed to believe that this advanced race, home to food machines and air machines and super-telescopes, a people with enough resources to pave a city in lead sheeting -- these guys could only come up with a puny rocket able to carry ONE FREAKIN' PASSENGER to safety? Ah, the Silver Age!
And as Argo City sinks slowly into the west, we leave this flashback in midflash, to be continued on Friday. Supergirl's rocket is on its way to Earth, and next she will try to explain to Karen Blair that it couldn't possibly have caused that deadly fire. After all, Supergirl is one of the GOOD GUYS.