Helloween’s original singer, Kai Hansen, lasted for exactly one full-length album: 1985’s Walls of Jericho, which ranks among power metal’s fastest and most influential albums. The German group retained him as a guitarist, and he finished his flawless tenure with the epic Keepers of the Seven Keys, Parts One and Two. Having telegraphed his desire to leave with the Part Two track “I Want Out,” Hansen founded Gamma Ray in 1988. Meanwhile, Helloween struggled through some bizarre albums (Pink Bubbles Go Ape, which contained “Heavy Metal Hamsters”; the genre-hopping Chameleon, which offered a swing number called “Crazy Cat”; Metal Jukebox, which involves an Abba cover) before a career resurrection that culminated in 2005’s Keepers of the Seven Keys: The Legacy. Helloween avoids its experimental missteps during its current set lists, which focus on progressive compositions with mythicized lyrics and harmonic speed-metal solos. Hansen’s operatic wail performs double duty during Helloween/Gamma Ray dates: He reunites with his old bandmates to reprise their classic material during the encores.

Fri., Oct. 3, 7 p.m., 2008