When three tall, dark strangers appear virtually out of nowhere on the riverbank in Cambridge where John's father is a professor--and even though their strange arrival coincides with some momentary dizziness and dislocation for John and his friend--it does take him a remarkably long time (more than half the book) to realize that the newcomers are visitors from the future. By then, John's older brother Ben and Katherine, the young girl of the trio, have fallen in love--so overwhelmingly that they run away to avoid her inevitable ""recall"" to her own time. Most of the rest traces John's pursuit of the couple, to warn them that Katherine will die if she doesn't return that night, and at the end the cast assembles for a sad farewell dinner while the lovers say their last goodbye in private. Townsend handles the time travel neatly but without teasing the imagination; what's left, then, are some temporary relationships--Ben and Katherine's sentimental, yet peculiarly wan love story, John's sympathy for Katherine's mother--and some conventional comic relief in the person of Katherine's childish father, turned loose in an imperfect world with alcohol, barmaids, and automobiles. Disappointing.