Saturday is set during the course of a single day in the life of Henry Perowne, an accomplished neurosurgeon living in an upmarket area of London.

Saturday, 15 February 2003 starts with Henry wide awake and in a state of inexplicable euphoria, some hours before dawn.  Standing at the window of his bedroom, he notices a plane coming in to land, apparently on fire. Henry is transfixed by the sight, reminiscent of the recent events of September 11th.

In the kitchen he chats with his 18 year old son Theo, an up-and-coming blues/jazz musician and the only member of the family not to follow a traditional and disciplined life path. Afterwards Henry slips back into bed besides his wife Rosalind. He reminisces about how they came to be together. She wakes and they discuss the new day - their daughter, Daisy, and Rosalind's father are both flying in that afternoon. 

Henry plans to start his day with a game of squash against old friend and rival, Jay. Outside, crowds are gathering for the protest rally against the invasion of Iraq. Henry struggles to decide which side of the debate he stands on. On the news, "his" burning plane story now ranks second to the protest.

Delayed by the thousands of protesters clogging the streets of the capital, Henry takes a shortcut down a closed road. A red BMW pulls out into his path and they collide. The other car is driven by Baxter, a streetwise and somewhat unhinged younger man.  With his two accomplices, he attempts to extort money from Henry, even threatening violence.  Henry picks up on the fact that Baxter is suffering from a neurological disorder, and humiliates Baxter by drawing attention to it.  He is able to slip away, but the encounter disturbs him and his squash game and mood suffer. 

On his way home, Henry picks up the ingredients for the fish stew he plans to make later for the whole family. His father-in-law, John Grammaticus, a famous poet known for his excessive consumption of alcohol and his unpredictable moods, will be joining them. He and Daisy have been estranged for months, and this reunion is intended to bring them together again.  Whilst driving he thinks he spots a red BMW in his rearview mirror but dismisses the possibility that it is Baxter. 

Back home, Henry has a quick lunch in the kitchen with Theo and mentions the incident. Theo cautions him - making a man like Baxter lose face in front of his crew could have potentially serious consequences. Henry leaves to visit his mother in a rest home. 

Lillian Perowne, once a strong, capable woman and a champion swimmer, is now senile, and seeing her depresses Henry.  He returns home in time for the arrival of his daughter, who seems more emotional than usual. Grammaticus arrives, and they are enjoying pre-dinner drinks when Rosalind walks in... with Baxter and associates in tow. 

Baxter has come to exact revenge for his earlier humiliation. During the ensuing confrontation, Grammaticus is punched in the face and Daisy is made to strip naked. Her disrobing reveals that she is four months pregnant. This saves her from being raped, and instead Baxter insists she read one of her poems. She instead recites one Grammaticus taught her as a child: Dover Beach by Matthew Arnold. Baxter is unaccountably enraptured by her recital, and agrees to accompany Henry upstairs to view some "research" into his neurological condition. His associates are unimpressed, and leave him to it. Theo and Henry manage to throw Baxter down the stairs, knocking him out.  An ambulance is called and he is taken to hospital. 

Later, Henry is called to the hospital for a head injury case: Baxter. He takes the job, saves Baxter's life, and comes to forgive him. He returns home and slips into bed beside Rosalind.  As Sunday dawns, Henry drifts off to sleep, his eventful Saturday finally over.