This is a very well-crafted murder mystery executed quite ingeniously. A mathematical physicist - Jan Friede - sets up a system of 20+ equations which eliminate the time variable from Einstein's equations (in General Relativity, unstated) and apparently form the Theory of Everything the "Ultimate Reality". He begins to grasp that our Reality is a projection of a timeless Reality much larger than ours - indeed, a reality created by a "Mathematical Mind" - and that we are just shadows as in Plato's allegorical cave.
He implements the equations on the "Intergraph", "a complicated machine with numerous keys and lever bars for solving intricate equations of the calculus". Just as he types in his final "equation 20b", the fatal equation, - he dies mysteriously. No one knows how and there is no evidence of foul play. The detective calls in his mathematician friend, George, who moonlights as a sleuth (his great tagline was, "Listen, old tube, I'm a mathematician, but I'm not a wizard"). George figures out that the murder was committed by the victim's friend (Dr. MacMillan), himself a mathematician. Evidently, MacMillan understood the implications of the system of equations and did not want the world to know the secret of our ultimate existence ("...an idea that would plunge the world in chaos if carried to completion. There are things unseen in this world that present primitive man must not glimpse. He must be content with the shadows of reality on his cave walls"). To kill Jan, he removes the insulations on the Intergraph keys in a particular pattern so that when equation 20b is entered by anyone on it, a massive electrical pulse stops the operator's heart; that's how the mathematical equation became fatal for Jan.
George stumbles on to the truth when he compares the equations on his copy to the one taken by the police right after the crime; a minus sign was changed to a plus by MacMillan to ensure that no one else got killed on the Intergraph. In the end, MacMillan jumps into a machine based on these equations in the hope that it will transport him to a Timeless state of Ultimate Reality.