Robert Plant's Lullaby...

Will Hermes/RollingStone

We'd like a Zep reunion as much as anyone, but it's easy to understand Robert Plant's resistance. Reinventing himself on his 2007 Alison Krauss collab, Raising Sand, and on projects with ex-girlfriend Patty Griffin, the singer made some of his best work ever. His latest returns to the world-music fusion of 2005's Mighty ReArranger with his Strange Sensation band (now reconfigured as the Sensational Space Shifters). It lacks the focused grace of his country experiments, but this much is true: It's Plant's hardest-rocking set in a decade. And it makes you wonder.

"Little Maggie," the album keystone, flips an Appalachian folk song (and bluegrass standard) with African space-funk grooves and hoedown-worthy griot ritti fiddle from Gambian bandmate Juldeh Camara. Heavier tracks are less convincing. "Embrace Another Fall," the sort of hybrid Peter Gabriel mastered, veers between boutique-hotel-lobby mixtape fodder and half-baked "Kashmir" gestures. "Pocketful of Golden" is a Houses of the Holy-style Celtic folk rocker that could use some Jimmy Page Les Paul shredding. Plant's Americana detour has made him a better, more nuanced singer. Here, coming home to something like progressive rock, it's hard not to think of his other band. Keep hope alive.