A splendid depiction of one the few true migratory butterflies, which journey from the coast of Maine to the forests of El Rosario, Mexico, and Pacific Grove, California, where they gather by the millions to winter. Lasky and a group of children observe the monarch in Maine, then travel to Mexico and California to see the species in its other habitat. Impressive detail, including the life cycle, renders the science adventure even more exciting; Lasky also covers naturalists' efforts to protect the monarch's habitat and the annual ``Friends of the Monarch'' parade in Pacific Grove (``Butterfly Town, USA''), when townsfolk dress as butterflies, flowers, and caterpillars, and raise money to save the few butterfly groves. Excellent color photos throughout enhance a book that truly captures the wonder of these intrepid creatures. (Nonfiction. 7+)