Blood Knot is a play about two brothers, who live in a one-room shack in a crumbling section of Port Elizabeth, South Africa. They are different in temperament, but they reaffirm and support each other. Morris is a light-skinned colored man, Zachariah is a black man. They are half-brothers, who have the same mother. They have shared the same one-room shack for about a year.

Zachariah works as a gatekeeper at a park. His job is to keep black people from coming into the whites-only park. He has footsores, from having to stand on his feet all day. Morris prepares hot water for Zach to bathe his feet. Morris has been saving the money that Zach earns, so that they can buy a two-man farm.

Meanwhile, the lonely Zachariah has struck up a pen-pal relationship with a white girl, and entertains fantasies that she might fall in love with him. The more level-headed Morris tries to disabuse Zachariah of such notions, and warns him that in segregated South Africa, such a relationship can only mean trouble, especially since the girl has indicated in letters that she has a brother who's a policeman.

The girl says in her next letter that she is coming to visit Port Elizabeth, and that she wants to meet Zach. She does not know that he is black. Zach suggests that, since Morris is light-skinned, he should take Zach’s place, and pretend to be a white man. Zach spends their savings on a suit of clothes for Morris, telling him that he should go to see Ethel.

In the end the girl decides not to visit. Morris and Zachariah will, apparently, remain together for many unhappy years to come, needing each other, but unable to bridge the gap brought about by their respective skin tones.