...exiled to the Phantom Zone by the sorceress Selina, Supergirl encounters Zaltar, her eccentric friend from Argo City whose machinations led to the loss of the Omegahedron, a Kryptonian power source now in the possession of Selina.
To ensure Kara returns the Omegahedron to Argo City, Zaltar leads her to the one, incredibly-dangerous, way out of the Phantom Zone...
The sequence with Selina's evil doppleganger did not appear in the final film.
In the movie, the monster she battled earlier reappears and almost destroys her before she sends it, Selina, and Bianca into the Quantum Vortex.
Writer Joey Cavalieri and artist Gray Morrow were working from earlier drafts of the script, and included elements that either were not filmed at all or were cut from the American theatrical version of the movie.
The DVD/Blu Ray versions (there are several) all contain bits and pieces of story that were not in the American release.
The most complete version is the HTF novelization by Norma Fox Mazer, a noted juvenile book author of the 70s-90s.
Here's the Gray Morrow original art for the final page of the story....
And watch Supergirl on CBS for the adventures of the new live-action Kara Zor-El!