Beyond belief or understanding:
— At incredible speed.
— The book's plot is simply incredible.

synonym: unbelievable.

Roget 31: great; greater etc. 33; large, considerable, fair, above par; big, huge etc. (large in size) 192; Herculean, cyclopean; ample; abundant; etc. (enough) 639 full, intense, strong, sound, passing, heavy, plenary, deep, high; signal, at its height, in the zenith.    world-wide, widespread, far-famed, extensive; wholesale; many etc. 102.    goodly, noble, precious, mighty; sad, grave, heavy, serious; far gone, arrant, downright; utter, uttermost; crass, gross, arch, profound, intense, consummate; rank, uninitiated, red-hot, desperate; glaring, flagrant, stark staring; thorough-paced, thoroughgoing; roaring, thumping; extraordinary.   ; important etc. 642; unsurpassed etc. (supreme) 33; complete etc. 52.    august, grand, dignified, sublime, majestic etc. (repute) 873.    vast, immense, enormous, extreme; inordinate, excessive, extravagant, exorbitant, outrageous, preposterous, unconscionable, swinging, monstrous, overgrown; towering, stupendous, prodigious, astonishing, incredible; marvelous etc. 870.    unlimited etc. (infinite) 105; unapproachable, unutterable, indescribable, ineffable, unspeakable, inexpressible, beyond expression, fabulous.    undiminished, unabated, unreduced^†, unrestricted.    absolute, positive, stark, decided, unequivocal, essential, perfect, finished.    remarkable, of mark, marked, pointed, veriest; noteworthy; renowned.    ... show more

Roget 471: impossible; not possible etc. 470; absurd, contrary to reason; unlikely; unreasonable etc. 477; incredible etc. 485; beyond the bounds of reason, beyond the bounds of possibility, beyond the realm of possibility; from which reason recoils; visionary; inconceivable etc. (improbable) 473; prodigious etc. (wonderful) 870; unimaginable, inimaginable^†; unthinkable.    impracticable unachievable; unfeasible, infeasible; insuperable; unsurmountable^†, insurmountable; unattainable, unobtainable; out of reach, out of the question; not to be had, not to be thought of; beyond control; desperate etc. (hopeless) 859; incompatible etc. 24; inaccessible, uncomeatable^†, impassable, impervious, innavigable^†, inextricable; self-contradictory.    out of one's power, beyond one's power, beyond one's depth, beyond one's reach, beyond one's grasp; too much for; ultra crepidam [Lat.].    ... show more

Roget 473: improbable, unlikely, contrary to all reasonable expectation; wild [Coll.], far out [Coll.], out of sight [Coll.], outtasight [Coll.], heavy [Coll.].    rare etc. (infrequent) 137; unheard of, inconceivable; unimaginable, inimaginable^†; incredible etc. 485; more than doubtful; strange, bizarre (uncomformable) 83.    ... show more

Roget 485: unbelieving; skeptical, sceptical.    incredulous as to, skeptical as to; distrustful as to, shy as to, suspicious of; doubting etc. v.. doubtful etc. (uncertain) 475; disputable; unworthy of, undeserving of belief etc. 484; questionable; suspect, suspicious; open to suspicion, open to doubt; staggering, hard to believe, incredible, unbelievable, not to be believed, inconceivable; impossible etc. 471.    fallible etc. (uncertain) 475; undemonstrable; controvertible etc. (untrue) 495.    ... show more

Roget 870: surprised etc. v.; aghast, all agog, breathless, agape; open-mouthed; awestruck, thunderstruck, moonstruck, planet-struck; spellbound; lost in amazement, lost in wonder, lost in astonishment; struck all of a heap, unable to believe one's senses, like a duck ion thunder.    wonderful, wondrous; surprising etc. v.; unexpected etc. 508; unheard of; mysterious etc. (inexplicable) 519; miraculous.    indescribable, inexpressible, inaffable^†; unutterable, unspeakable.    monstrous, prodigious, stupendous, marvelous; inconceivable, incredible; inimaginable^†, unimaginable; strange etc. (uncommon) 83; passing strange.    striking etc. v.; overwhelming; wonder-working.    ... show more