Rising politician Ivor Tresham meets Hebe Furnal at a fundraiser for a charity her husband manages. They begin a brief affair which ends with her death in a car accident during a faked kidnapping - the titular 'birthday present' being an evening of 'adventure sex.' After an initial media frenzy focused on Hebe's husband, the press and the police decide that the real target of the kidnapping was a millionaire's wife who they hound into a nervous breakdown. Meanwhile, Joe and Iris Delgado observe while her brother tries to find out who knows that he was the mastermind behind the kidnapping.
Over the next three years, Ivor gradually becomes involved with the kidnapping survivors. A combination of guilt and fear leads him to track down Dermot Lynch, who survived the accident with severe brain injuries, and Juliet Case, the ex-girlfriend of Lloyd Freeman who died in the accident. Juliet's motivation for her affair with Ivor seems clear, but it's not - we see only what Joe Delgado sees and the depths of her feelings remain obscure until the climax. Similarly, we never truly know what Dermot Lynch's mother and brother know or believe, or whether Ivor's fear of blackmail is real or imagined.
Vine intersperses Jane Atherton's diary entries with Delgado's straight narrative. Hebe was Jane's best friend, or rather only friend. She's a lonely, rather mousy woman who was born to be used, first by Hebe and later by Hebe's husband Gerry, in part because she lets herself be used. She drifts through life with a sense of self pity and a bit of self-sabotage, and although she never crosses paths with Ivor, her actions indirectly set up Ivor's final act.
The car crash which was the ultimate end of the 'birthday present' is a good metaphor for the book - it's sort of like watching a slow-motion car crash. The events unfold slowly over 4 years, against the backdrop of the Conservative Party's gradual fall from power and a movement against tabloid sleaze. Ivor's fate was totally unexpected, but completely supported by what went before. I've read most of the books Ruth Rendell has written under the name Barbra Vine, and I'd rank this one second, directly and barely below A Dark Adapted Eye.