A shape-shifting construct created by malevolent extra-dimensional beings arrives on Earth as a stowaway on a freighter from Mars, having disguised itself as various inanimate objects during transit.
The Martians read its mind and were appalled by its intentions, but could not physically destroy it, so they imprisoned it in a massive vault of ultimate metal secured with an ieis time-lock set to only release at the unimaginably distant End of Time.
The construct's Controller strikes a conciliatory tone, apologizes for the murders, and explains that his comrades are merely trying to rescue their colleague Kalorn, a scientist who had discovered how to bridge their two "number spaces."
It again tempts Brender with the wealth of the lost city, this time adding that it has read his wife's mind and learned she still loves him, and would return to him if he regained his station.
Brender and the construct arrive in Li and alight on the nearly-buried roof of the Vault, beside the lock: a flow of flickering ieis bridging two terminals of ultimate metal.
Brender rapidly solves the problem, explaining to the Controller that only a finite quantity must be physically subtracted from the flow of ieis to make the infinite largest prime factorable.
Brender pityingly calls it "poor Frankenstein," then uses the Controller-enhanced ship to almost-instantly return to Earth, where his wife - still unaware that the treasures of Li have once again made him fabulously wealthy - meets him tearfully at the spaceport and tells him she has never stopped loving him.