Ever since the demise of screamo progenitors At the Drive-In, guitarist Omar Rodriguez-Lopez and vocalist Cedric Bixler-Zavala have been on a mission to epitomize all that is prog rock with the Mars Volta. The band has met this goal with steadily decreasing degrees of execution, from its 2002 Mahavishnu Orchestra-meets-Rush debut Tremulant to this year's Renaissance Fair-appropriate acoustic album Octahedron. While the band's discs meander enough to practically eject themselves, its concerts are a feat to behold. Bixler-Zavala flops around like Iggy Pop impersonating a salmon out of water, and Rodriguez-Lopez shreds while tap dancing on pedals, his drumsticks whizzing by in a nearly incomprehensible blur. It's then that the Mars Volta transcends its prog tag and actually does something progressive.