Tristram lives an 'alternative' lifestyle with his father. Their remote house on a peninsula has a generator for electricity and a bath in the garden. Tris's mum left, having tired of this way of living and, taking the car with her, went to Sidney. Tris misses her, but remembers even more vividly than his mum the family who lived next door to them when he was small - Dearie, who was his child minder, and Cecily, her daughter, who protected the young Tris. His daydreams involve these people, but he also dreams about his mum and his hopes of hearing from her. He plays imaginative games with a character he calls Selsey Firebone, a sort of SAS commando type, and these games involve the underrunners of the title - tunnels under the tussocks of grass in which Tris can hide. His father has begun to show interest in a woman who comes regularly to visit and whom Tris is convinced is out to get his dad. Victoria and her little girl Rosie are friendly and pleasant, but Tris isn't happy about their invasion of his life. He meets a girl staying at the local children's home to whom he has a strange attraction. She is worried about a man who she believes to be after her, and while Tris thinks she is overreacting, it soon develops that she is quite right. The man is her father, a dangerously unstable character, who kidnaps the two children and carries them off to a house in the city - the house next door to where Tris used to live. The girl is Cecily Tyrone, his protector from years before, who has called herself Winola until this point, and in Tris's mind, Cecily Tyrone has become 'Selsey Firebone' his imaginary friend. It is a tribute to Miss Mahy's ability that the coincidence of the children's having met again is entirely believable. Cecily is a wonderful, tough character, who, one feels, will pick herself up from her dreadful family and manage some sort of life. The climax is a real nail-biter! But after all the fireworks are over and Tris wants his dad to adopt Cecily, she refuses. With her mum in a mental hospital and her dad in a prison hospital, she says, 'I'm not one of those too big kittens someone's dumped.' She will stay in her foster home until her mum is out of hospital. A survivor is Cecily. An enthralling read.