England, 1900. Twelve-year-old Leo spends the summer with his wealthy classmate Marcus at his family's grand house in the Norfolk countryside. He is befriended by Marcus' beautiful older sister Marian, who takes him on a shopping trip and buys him new clothes, winning his confidence. On a swelteringly hot day, they take a trip to the lake, only to discover the local farmer, Ted Burgess, already swimming there.
When Marcus falls ill with measles, Leo is left alone and walks by himself over the fields to the neighbouring farm. After injuring himself sliding down a haystack, he is nursed by Ted, who asks him if 'he will take a message to Marian in return. A few days later, Leo, sent on an errand to find Marian, comes across her emerging from the bushes in disarray. She begs Leo to take a message back to Ted, and so he becomes their 'go-between', entrusted to carry secret letters between them. One day, Leo is given an unsealed letter, which he reads. He realises that the two of them are meeting in secret, even though Marian is soon to become engaged to the likeable Viscount Trimingham.
At a village cricket match several days later, Ted proves himself to be a powerful hitter, until he is caught out by Leo. At the post-match celebration, Ted is entreated to sing on stage and is accompanied on the piano by Marian. He sings a song about a 'lucky man' who wins the heart of a fair and rosy-cheeked woman.
While Marian and Leo are play fighting, her mother catches sight of a letter, intended for Ted, which Marian pretends she has asked Leo to deliver to her nanny. Her mother presses Leo and forces him to admit that he has taken letters for Marian before, though he cannot remember the way to the nanny's house. Several hours later, at Leo's birthday celebration, Marian is late arriving and a carriage is sent to collect her from the nanny's home. A message comes back that she has not been there all day. Marian's mother flies into a rage and, grabbing Leo by the arm, she forces him to take her to Ted's farm, the real destination of the letters. Following the sounds of heavy breathing, she drags Leo into a hayloft where they stumble across Ted and Marian in the sexual act. She tries to bury Leo's face so he cannot see, but it is too late.
Fifty years later. We see an aged Marian and Leo as an older man. She entreats him to do one last errand and visit her grandson (who resembles Ted Burgess) to tell him the whole story. We learn that Ted shot himself after their discovery, and infer that Marian married Lord Trimingham.
