Barry spends the majority of the episode inside the S.T.A.R. Labs pipeline because, according to Danielle Panabaker, it was filmed concurrently with the "Elseworlds" crossover, so Grant Gustin wasn't able to appear in much of this episode.
First appearance of Silver Ghost, who, in the comics, was a Nazi leader on Earth-X. Raphael Van Zandt was double-crossed by Rod Reilly (Firebrand I), a resistance leader who posed as a Nazi sympathizer. When the war ended on Earth-X, Van Zandt traveled to Earth-1 in an attempt to track down Reilly. During his inter-dimensional trip, Van Zandt received his powers.
Barry asks how Joe's "back is?" a reference to the fact that Jesse L. Martin has been recovering from a back injury, forcing him to miss several episodes.
In the comics, Silver Ghost can transform humans and objects into silver making them under his control.
At Weather Witch's trial, Cecile mentions the dates November 20th and the 27th a reference to when the the episodes the The Icicle Cometh (2018) and O Come, All Ye Thankful (2018) the latter featuring Weather Witch came out.