The brother-and-sister duo Marry Waterson and Oliver Knight are the children of the late British songstress Lal Waterson. Knight has been either recording or producing music since he was 13. Waterson's career actually began with the duo's 2011 album, The Days That Shaped Me. While that recording proved interesting, it was also predictable as an intimate, and largely acoustic, singer/songwriter affair. Despite her heritage, Waterson's songs are intuitive. Not playing an instrument, she writes entirely in her head; she then hums or sings melodies to Knight, who orchestrates and arranges them. This makes them collaborators in a very real sense. Hidden reflects the increasing comfort the siblings have with one another in the recording studio. More importantly, it reveals remarkable growth in their songwriting prowess. Hidden is more extroverted, musically diverse, and ambitious all around, its songs character-driven rather than just first-person narratives. While some of these songs evoke earlier musical eras quite authoritatively -- the lazy ragtime in "Going Going Gone," the Piccadilly Circus-era stage music in "Scarlet Starlet" -- others songs, such as "Benign" (which features uncle Martin Carthy on acoustic guitar) and "Love Song to a Lyric," readily reveal the Brit-folk lineage but with modern twists in melody and articulation. The real surprise, however, is a more rockist dynamic in some of the set's best cuts. "I'm in a Mood," "Gormandizer," and closer "Starveling" all feature Knight's electric guitar playing prominently (with a backing band to boot). Obviously influenced by Richard Thompson and David Gilmour, his chord voicings, single-string lead playing, and sonic effects are textural, precise; they're never flashy or intrusive yet they have heft and drama. A fine example is in "I Won't Hear" -- which has to be the only accordion-driven ska tune in existence -- where his stinging Stratocaster lines in the instrumental break add a more authoritative dimension to Waterson's crystalline contralto. Another highlight is "Russian Dolls," which also features cousin Eliza Carthy, Barry Cooper, and Lester Simpson on backing vocals. It's based on a traditional round, but its use of blues and doo wop against that backdrop is surprising and unique. Hidden is not merely a second step for this duo, but real deepening in a highly individual sound.