You really do have to watch out for the quiet ones.

Radiohead’s Thom Yorke’s voice rarely gets above a warm, lullaby-ready croon on the band’s ninth album “A Moon Shaped Pool” (XL), but he’s generally singing about dark topics designed to keep you up at night.

While “A Moon Shaped Pool,” released as a Mother’s Day surprise on Sunday, features no major shifts musically, lyrically Yorke really does have his knives out and sharpened this time around.

“Sweet-faced ones with nothing left inside that we all can love,” Yorke chants at the beginning of “Identikit,” one of the most upbeat-sounding songs musically on the album. Then the chant of “Broken hearts make it rain” over layers of electronic distortion begin and the talk of “pieces of a wreck of mankind” start and the method becomes clear.

Radiohead doesn’t just want listeners to hear these messages of isolation and angst, they want to drum them into fans’ heads as efficiently as a Rihanna chorus.

The first single “Burn the Witch,” with its frantic string section arrangements and orchestral flourishes, is trying to keep the idea of paranoia and “low-flying panic attack” in your head for the duration of the song, conjuring up a world where you can trust no one and should “shoot the messengers.”

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When Yorke gets personal, the lyrics are even more wrenching and the accompaniment all-the-more beautiful. On “Glass Eyes,” a ballad that truly could have, with a few tweaks, come from Adele’s “25” album, Yorke worries about fitting into daily life, singing, “Panic is coming on strong, so cold from the inside out.”

It’s part of “A Moon Shaped Pool” ’s considerable charm that darkness has never sounded quite so attractive.