DI Lewis returns to Oxford after two years' secondment to the British Virgin Islands, where he was sent to recover from his wife's death.
He is reluctantly assigned by his new boss, DCS Innocent, to the murder of an Oxford mathematics student who is shot while participating in a sleep study.
When two other murders occur, Lewis must decipher a cryptic clue left in an old case file by his former boss, Chief Inspector Morse: "Polo not king after all".
Several other references are made to Morse: on his leaving London Heathrow Airport, Lewis is nearly run over by a red Jaguar Mark 2 (which also reminds him of his wife Val’s death in a hit-and-run); he knows the word "bruxism" (tooth-grinding) as a result of spending "15 years working for a crossword fanatic"; Trudi Griffon knew Morse and says: "He was a man you could talk to"; Griffon's daughter Jessica Pollock is being examined for the "Endeavour Award" which had been endowed by an anonymous donor for music that spoke to the soul.
Patsy got drunk or drugged with the Sons of the Twice Born, and they decided to murder her in order to take out her adrenal gland and extract the adrenochrome, which was reputed (by Hunter S. Thompson) to give a very good high.
The couple seem to be reconciled, but in reality Tina and Anne are manipulating the Sons of the Twice Born to murder each other: This leaves Platt outstanding – and he is the only one who knows where Patsy is buried.
Lewis and Hathaway consult Professor Margaret Gold, who immediately deciphers them as telephone numbers written in Greek numerals.
DI Lewis is less than pleased when he and DS Hathaway are assigned to protect Nicky Turnbull, a former convicted cybercriminal and "Rock and roll hacker" turned successful author.
He is met by a welcoming committee composed of Caroline Morton, president of the Union; Jo Gilchrist, unscrupulous student journalist with ambitions to be a tabloid editor who intended to expose a professor's exam scam; Stephen Gilchrist, her brother, who is a serious scholar; and David Harvey, a rugby player who has had to retire because of an injury.
Turnbull gives his lecture and afterwards treats his audience to champagne in his hotel, which is housed in the converted prison in Oxford Castle.
Things take a more serious turn the following morning when Jo is found strangled in the hotel room next to Turnbull’s, which had been booked in Lewis’s name.
Chief Superintendent Jack Cornish, an ex-colleague of Robbie's, goes missing and a recently embalmed body is discovered near an isolated farm track.
Lewis takes the awkward, young but bright DC Gray (Babou Ceesay) under his wing to investigate the case.
It transpires that Cornish has gone to ground in a cottage in Croatia, where he manufactures crystal meth, which is smuggled into Britain inside corpses being repatriated to undertaker Brian Miller.
Hathaway returns from making the Camino de Santiago pilgrimage, and he starts work on his first murder case as an inspector with the help of a new partner, DS Lizzie Maddox.
Unwilling to delegate responsibility to an increasingly frustrated Maddox, he is overwhelmed by the work involved, and requests help from Innocent, who persuades the now-retired Lewis to come back on a one-year contract (to the annoyance of Laura Hobson, who had hoped for a quieter life with her retired partner).
Lewis, Hathaway and Maddox's abilities as a team are severely tested when they investigate the brutal stabbing of American classics scholar Rose Anderson.
Garwood later meets a violent end in his office filled with antique astronomical instruments, including a telescope that had previously belonged to Isaac Newton.
Meanwhile, questions arise regarding the classics professor Simon Flaxmore, who had made his name by apparently rediscovering the lost Euripides play Alcmaeon in Corinth (Ἀλκμαίων ὁ διὰ Κορίνθου, 405 BC).
Hathaway works out that the "rediscovered" play is a fraud, as it refers to the "two lions of Nemea", whereas the constellation of Leo Minor was only so named by Johannes Hevelius in 1687.
Jennie Brightway and her husband Paul, desperate to engender a saviour sibling via in vitro fertilisation, have been struggling to find the money for this expensive procedure, but Anderson had donated a large sum to help them.
Lewis and Hathaway realize that student Sam Langton, who is in a relationship with Carina Beskin, is in reality Francis Fisher’s younger son John, who is wreaking vengeance.