Aston, a reserved and quiet man, lives alone in a top-floor cluttered room of a small abandoned house in a poor London district. He befriends and takes in Mac Davies (Donald Pleasence), an old derelict who has been fired from a menial job in a café. In time, Aston offers him a job as caretaker of the house.
As time goes on, Mick, Aston's brother, becomes increasingly irritated by Mac Davies, who is a taunting, quasi-sadist. When his brother is away, Mick countermands his orders and harasses the derelict. Despite this, Mac Davies remains loyal to Aston and continues to live in the house.
One day, Mick comes home to find that Mac has gone missing. He immediately assumes that Mac is dead and begins to search for him. However, he soon realizes that Mac is alive and hiding in the basement. Mick confronts Mac and threatens to kill him if he doesn't leave the house. Mac refuses to back down and challenges Mick to a fight.
Mick, determined to prove himself as a better man, agrees to the challenge. They engage in a physical altercation in the basement, with Mick eventually overpowering Mac and forcing him to leave the house.
As Mick returns home, he is surprised to find that Mac has not left the house. Instead, he finds Mac sitting on the front steps, waiting for him. Mac explains that he was hiding in the basement so that he could watch Mick's reaction when he came home empty-handed. He then reveals that he had been living in the house all along, having been evicted by Mick's parents.
Mick is initially shocked by this revelation but eventually accepts it. He apologizes for his behavior towards Mac and promises never to treat him like a derelict again. Mac forgives Mick and they become friends once more.