The Cell, directed by Tarsem Singh, is at least partially a Girls Underground story if looked at from the right angle, and an interesting one at that.
Catherine is a psychologist who uses special technology to go inside the mind of her young patient who cannot communicate any other way. She is recruited by the FBI to use the same approach on the comatose serial killer Carl Stargher, in order to find out where he has stashed his latest victim. So the otherworld she enters is entirely internal – inside the adversary’s mind. Thus, the whole world is under his control. But while one might see the female victim as the customary person that the GU must rescue, another perspective is that it is Stargher himself, or rather his child self who inhabits his internal world along with a larger-than-life terrifying Adversary version of him (as Catherine says, he sees himself as the “king of a very, very twisted kingdom”).
Catherine temporarily forgets herself in Stargher’s world – not only believing in the reality of the internal landscape but being remade into his evil self’s willing accomplice, similar to how Lily in Legend or Helena in Mirrormask are darkly transformed by their adversaries. The FBI agent on the case must join her in Stargher’s mind to remind her who she is – briefly becoming a companion of sorts. However, once the agent finds a vital clue and rushes off to find the victim, Catherine re-enters Stargher’s world on her own (without even her team to monitor her well-being) and confronts him alone. She defeats the adversary and humanely ends his child-self’s suffering, while the FBI agent simultaneously rescues the kidnapped girl just in the nick of time.