When the family at the center of The Spellcoats is on their journey down the flooded River, they arrive at a place where another river meets it. They have been wanting to stop and camp somewhere, under shelter, so that they can take better care of their disabled brother Gull, and so that they can get dry and get more food. However, they keep coming to islands that are sheltering dark-haired people like the villagers of their own village, and each time those people see the children, with their light hair, they start shouting "Heathens!" and throwing stones at the children.
Daughter Tanaqui keeps dreaming that her mother appears to her and each time says "Riversmeet" as if it's very important, and she becomes convinced that they should stop whenever they find another river, so she argues with her brother Hern, who's steering their boat, and he eventually gives in. When they stop, a plainly dressed, tanned young man who appears to be a heathen from his light hair approaches them and offers them shelter. He has a gentle, laughing look, and carries no weapons, so they agree. This is Tanamil.
The four children spend an unknown length of time with him after he turns Gull into a reddish statue made of mud, through the use of a reed pipe. He talks to each of them separately and together, feeds them, lets them sleep in the rugs by his fire, and teaches each of them something that person needs to know. But after a quarrel of some kind with Robin, who refuses to stay with him when the other three continue down the river, Tanamil disappears and they leave in their boat. They realize that although they arrived at Riversmeet in winter, they are now sailing through Spring, and everyone is very confused.
Later Tanamil reappears in the story, and again helps the family, although again they are not sure of his intentions or why he is so unusual.