Dark matter is the secret stuff that makes up much of the universe. It's also the shocking stuff whispered about dons and students at Oxford, which Inspector Lewis and Sergeant Hathaway hear in excess as they try to get to the bottom of the suspicious fatal fall taken by Andrew Crompton, amateur astronomer and Master of Gresham College.
Crompton was obsessed with something having to do with the planet Venus. To hear tell, he was also obsessed with sex. But was his longtime mistress Dr. Ella Ransome, as claimed by the college's head porter, Roger Temple? Or perhaps it was Roger's wife, Babs, as alleged by Roger's father, Ted, who is succumbing to Alzheimer's disease and is Dr. Ransome's patient. Or could it have been astronomy lecturer Lady Gwen Raeburn, who is unusually tearful about Crompton's demise?
All of these potential partners suggest a vengeful spouse as killer, not to mention Crompton's own spouse, who likes to relax by squeezing off rounds at the college's rifle range.
As Lewis and Hathaway work their way through the suspects, which include two students who have been acting suspiciously, a gala memorial concert is taking shape. Police pathologist Dr. Laura Hobson is in the clarinet section and is enlisted to spy on the various orchestra members who figure in the investigation, notably Gwen; her husband, Sir Arnold; and the guest conductor Malcolm Finniston.
When a principal in the case is shot on the college's quad, no one can identify the sniper. And yet Lewis and Hathaway can't figure out how the homicide could have happened in broad daylight with witnesses all around — a mystery that, in fact, holds the key to both killings in this very dark matter.
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Warning: Contains significant plot spoilers
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Warning: Contains significant plot spoilers
While bicycling to work, head porter Roger Temple of Gresham College sees the master of the college, Andrew Compton, in an apparent assignation with beautiful Dr. Ella Ransome. Inclined toward petty blackmailing to make ends meet, Roger takes careful note.
Later, the overwrought Crompton attends confession for the first time in years, then leaves proclaiming, "Friday, 3:15, when I'll have an excess of joy!" That night he goes to the college observatory, where he is found the next morning at the bottom of the telescope stairs — killed by a fall. Police pathologist Dr. Laura Hobson notes scratches on his face, suggesting a struggle and therefore murder.
As Inspector Lewis and Sergeant Hathaway make inquiries, a picture emerges of an unusually incestuous college community. Roger believes that Crompton was having an affair with Dr. Ransome. But Roger's demented father, Ted, who was once porter himself, tells his physician, Dr. Ransome, that the master was secretly seeing Roger's wife, Babs, a janitor at the college.
Meanwhile, Babs has caught her nephew, Jez Haydock, a devoutly religious undergraduate, in bed with schoolmate Kate Cameron, a judge's daughter who soon learns she is pregnant. Kate's consultation with Dr. Ransome concerning an abortion infuriates Jez — leading to furtive behavior that casts suspicion on both of them in the murder inquiry.
Best friends with Crompton and his wife, Isobel, are the renowned composer Arnold Raeburn and his wife, Gwen, an astrophysics lecturer and intimate of Crompton on matters celestial. However, Gwen might have been close to Crompton in other ways too, as she proves to have been with visiting guest conductor Malcolm Finniston, a former pupil of Arnold's whose youthful, compromising photos with Gwen were stolen and stored away by Ted, until found by Roger.
Roger's attempt to blackmail Malcolm with the photos is discovered by Laura, who happens to be in the community orchestra preparing for a memorial concert in Crompton's memory. In the midst of this Peyton Place morass, a sniper shoots Dr. Ransome dead in the college quad!
What are Lewis and Hathaway to make of all this? Isobel seems the obvious suspect, since the two murders involved her husband and his presumed lover. But what does Crompton's declaration about Friday at 3:15 mean? On a hunch, the two cops show up at the observatory at the appointed time and find Gwen and Jez, testing a crackpot theory of Crompton's about the timing of Venus's transit in front of the sun.
It seems that Crompton was gradually losing his mind to a brain tumor; that his meetings with Dr. Ransome were not trysts but medical consultations; and that the real murderer was the one person who could carry a rifle across the quad in broad daylight without notice ... hidden with the mops and brooms of a cleaning cart. Namely, Babs.
Ted was right that Babs had been having an affair with Crompton, whose death happened inadvertently during an argument after he insisted to Babs that his wife must be told all. Then when Roger tipped off police that Crompton had been seeing Dr. Ransome, Babs had to kill her to hide the real truth.
Cornered by Lewis and Hathaway, Babs confesses while holding a rifle. Then she turns the weapon on herself. But Hathaway saves her for the jury.