Carl Sillmans, a merchant banker firm midway through an expected merger with Dinslow Chemical Industries, is thrown into turmoil when its manager, Charles Makepeace, is found murdered. His flask had been poisoned, and as Detective Superintendent Lance Thorne investigates, he finds that there're a surprising number of people who may have wanted him dead. His wife Isobel, for example, whom he was planning to divorce for his mistress, Stella Tyson. His work colleague, John Field, who learned that day he was to be denied promotion, preferring an American arriving later that day, Dan Maitland, son of a rival firm. And then there's the mysterious threatening phone call he received the evening he died that suggests something far darker is happening than anyone realises.

Field and Maitland travel to Dinslow Chemicals after noticing that one of the protesting activists was at a number of companies targeted and suspect he may be a plant. The duo are attacked, but Field is later contacted by the man, Nathaniel Mustard, by phone and arranges to meet him. However, unbeknownst to Field, Mustard is later ambushed in his car by masked men and carted away. Meanwhile, Makepeace's secretary Nina Novotny discovers in one of Makepeace's dictations a recording of a woman accusing him of being a murderer. She contacts Thorne, but finding out who the woman is will be only part of the problem — especially when he discovers that the woman has disappeared.

Field discovers Mustard's corpse in Tindell's Pond, a nuclear waste dump owned by Charles Tindell. Thorne has learned that Makepeace discovered the illegal practice and reported Tindell, who was later jailed and committed suicide, and suspects the woman in the recording was his wife. Field and Maitland confront a representative of Fane and Browning, a rival company over the fact that Mustard once worked for him and suspect he was hired by them to damage Dinslow Chemicals' chances in the merger bid. Maitland sees an old flame, Lady Bea Tarnigan, at the same hotel, but later Field becomes jealous of Maitland after he meets his mistress Kate MacRenny by chance at the tennis court and becomes interested in her. Thorne locates the widow Alison Tindell and her son Rupert, and learns that Rupert does not have an alibi for the time of Makepeace's death. Little does Thorne realise that his wife Millie plans some investigating of her own that could put her in grave danger.

Thorne is angry with his wife after her interference sees Roger Tindell go into hiding. And worse is to come as the hunt for his whereabouts threatens the life of his assistant. With Carl Sillman's takeover bid being referred to for investigations, Field floats the idea to his boss Sir Max Sillman to investigate Fane and Browning, but Sillman is reluctant to do so, unaware that Maitland is doing inquiries of his own via a colleague who works for them. Meanwhile, Thorne visits Sillman's astrologer Musquat Singh, who often clashed with Makepeace. Singh reveals that he received a mysterious call from someone asking to visit him, but he never arrived. Singh's assistant Krishna contradicts his alibi and mentions that he overheard Singh talking to a Scotsman. One of them is lying — but who? And why?

Roger Tindell stakes out Thorne's home, but his intentions are not what they think. Thorne also has his own men staking out by Musquat Singh and his assistant Krishna and later discover Krishna giving his restaurant father a brown package. Singh makes his own confession to Sillman. Isobel wants to resume her affair with Sillman and wants him to invest in her new business, but finds him less than compliant. Meanwhile, as Field looks into Fane and Browning dealings, Maitland is one step ahead and having discovered links between the company and Charles Tindell, he has a surprise in store for them at their business conference.

Thorne realizes that both Makepeace's mistress Stella Tyson and his wife Isobel could be lying about his marriage proposal to Stella and of the break in to his flat, but both are on the defensive. However, it also means that Makepeace's flask could've been poisoned earlier than thought. Carl Sillmans celebrate following Maitland's work in discrediting Fane and Browning and throw a party. Maitland brings Kate, much to Field's chagrin, but are salutations a little premature with Makepeace's killer still at large and Thorne ready to make his move?