The opera's subtitle comes from "Nothing is true and everything is allowed", which was the Assassins' secret doctrine according to Beethoven's acquaintance Joseph von Hammer-Purgstall.

The plot involves meserism, clairvoyance and psychic healing, mentioning Beethoven's connections with practitioners such as Johann Malfatti and Ludwig Schnorr.

"[6] The Scotsman called it "a brilliantly well-informed 200-year history of philosophy, science, music and mysticism, touched with an edge of Da Vinci Code hocus pocus," and remarked on "the sheer fun and narrative energy of Crumey’s writing, the skill and insight with which he conjures up each of his narrators from the repellent to the poignant, and the huge ingenuity with which he interweaves their stories.

"[7] The Herald (Glasgow) commented, "Beethoven's Assassins may be a gloriously multi-faceted puzzle-box of a novel, but even those anticipating a dense, abstruse intellectual exercise of interest only to literary theorists will find themselves drawn in by its well-drawn characters and emotional weight... Beethoven's Assassins is that refreshing thing, a novel of ideas with all the intrigue and momentum (and occasional red herring) of an absorbing mystery, underscored by a dark, ironic sense of humour.

"[8] Paul Griffiths wrote in Literary Review, "Crumey gives each of his chapters its own period and central character, and flips from one to another with the dexterity and humour of a champion juggler.

"[9] The Spectator judged Beethoven's Assassins "great cerebral fun, with its quantum physics, telepathy, time travel and fraying of fact and fiction.

"[12] Beethoven's Assassins features the Coyle family (Joe, Ann and their son Robert) from the fictional town of Kenzie.

The writer Heinrich Behring figures in Beethoven's Assassins as a disciple of George Gurdjieff, having previously appeared in Mobius Dick and The Great Chain of Unbeing.