Detective Comics #418 (DC, 1971 — Cover A)

The Creeper is unable to transform back into his Jack Ryder identity and is being driven insane by Dr. Yatz's serum. But Yatz's son Ishmael has the Creeper steal for him, promising to cure him of his condition, and Batman becomes involved with the case as a result. During the premiere of a new movie titled The Stepfather -- based on the true story of the 1920s crime kingpin known as Floyd Marcus -- premieres at a Gotham theater, Marcus (who is in attendance) is gunned down in cold blood upon his arrival. Recognizing the gunman as Marcus' stepson Mike, Barbara Gordon changes into Batgirl to search his estate for proof linking him to the murder.