Supergirl (1972)The Devil's Brother!

A mysterious book mailed to Linda Danvers begins draining her powers just as she and Superman chaperone a group of teens at an amusement park. When a young ward vanishes, Supergirl is pulled through a dimensional doorway into the domain of an alien tyrant.

Stripped of her strength, she must decide whether to trust her captor's bargain to get the girl back safely.