This episode had an even greater-than normal amount of sheer stupidity in the plotting.
So these cult people have a journal which gives them instructions on how to assemble a world-killer-maker.
(Stupidity #1) They should make a copy of it so their entire reason for existing doesn’t fail if it gets stolen
Then the defector steals it, and they blackmail James to get it back
(Stupidity #2) There’s often a situation in a show like this where the good guys are balancing the life of a hero, or a loved one, vs the fate of the entire world. That’s presented as a touch choice. Here they’re balancing a revelation about James so minor that by the end of the episode he’s decided he’s actually going to voluntarily make it, vs effectively giving a WMD to a terrorist cell. What an incredibly non-interesting choice, that they still make wrong.
(#3) How did the cult get tech good enough to defeat all DEO tech for tracking a van? They’re not supervillains or anything
(#4) What ever happened to just following a car? You know, the way government law enforcement agencies can do? Or maybe just have supergirl flying up high overhead keeping an eye on things?
(#5) Also, if you’re going to give up the girl and the book, how about giving up a shapeshifted J’onn instead of the real girl? (And if the plot required it, they could certainly whip up a deliberately incorrect but convincing copy of the book in a few hours.)
(#6) So Mon’El, who has nearly super-girl powers, is 10 feet away watching this ceremony take place, and he sees the crucial step happening, and yet somehow can’t stop a bunch of completely unpowered humans from completing the ceremony until it’s too late. Once the van arrives all he has to do is just go immediately subdue everyone, then there’s no more risk of ceremony, and they can search for the rock at their leisure (at which point it won’t yet have been used)
(#7) Evil mastermind Lex Luthor built a home security system where the cloaking device is disabled by smashing the control panel. I see a flaw in your plan there, Lex.
It’s like the writers just don’t care at all. It’s one thing for them to make a good effort and occasionally some cracks show up, since it’s obviously hard to tell stories about a being as powerful as supergirl. But, come on, you’re professionals. At least TRY to have any level of consistency or logic in people’s actions.