Brightmusic, the city’s shape-shifting classical ensemble with a boundless, dynamic repertoire, starts its new season Monday. The season’s lineup is as varied as one would expect, simultaneously comfortable and challenging.

“We’ve done something very different. This year, there will be no sixth concert, but a four-concert festival instead,” said David R. Johnson, Brightmusic president. Each concert will be performed twice, Mondays at Casady School, Tuesdays at the group’s ancestral home of St. Paul’s Cathedral.

“Twilight of Romanticism,” the first concert, will feature the Quartet for Piano and Strings in A minor, an unfinished piece by a then-teenage Gustav Mahler, written in the 1870s. This only surviving movement of his sole attempt at chamber music has been performed around the globe since the mid-1960s and reflects the self-doubt and old-before-his-time weariness that makes him a sort of spiritual forebear to all the doomed romantics who followed.

Alban Berg sometimes was criticized for the coldness of his work. He wasn’t a musician per se, so perhaps his built-in aloofness meant he couldn’t quite communicate his work, and he didn’t conduct, either. Brightmusic will interpret the Adagio from Chamber Concerto, rarely performed. One of the unique features of the piece is that the music reverses about halfway and makes its exit. Satanic madness? No, likely just a device Berg employed discretely to depict the comings and goings of a certain lady.

Austrian composer Carl Frühling’s Trio in A minor, op. 40, qualifies as the tall, dark stranger of the evening. Known mostly in his day as an accompanist, Frühling also was quite a talented composer.

“His work has been lost in the vapors of history,” Johnson says. “There’s an interesting story behind him, which you’ll hear at the concert. You know, the canon changes sometimes, and Frühling is an example of that.”

Johnson also noted that renowned cellist Steven Isserlis has performed Frühling of late, bringing the composer even later notoriety. The influence of Johannes Brahms and Richard Wagner can be heard easily by the novice, but Frühling’s own instrumental abilities serve him well.

Ernst von Dohnányi was something of a star in Hungary in the early 1900s, and was in great demand as a virtuosic pianist, conductor and teacher. Although he championed Hungarian nationalist composers like Béla Bartók, his own early work reflects the long shadow of Brahms. Brightmusic will perform his Piano Quintet No. 1 in C minor, op. 1.

Brightmusic artists who will perform “Twilight of Romanticism” are Gregory Lee and Katrin Stamatis, violins; Royce McLarry, viola; Tomasz Zieba, cello; Chad Burrow, clarinet; Amy I-Lin Cheng, piano.