Sea invertebrates carry most of the animal kingdom's evolutionary history, and this series builds its case around them: sponges, anemones, flatworms, mollusks, arthropods, jellyfish, and sea stars, filmed from tropical shallows to the deep sea floor. This episode opens the series by laying out the premise that these overlooked animals, not vertebrates, hold the record of how life diversified on Earth, using underwater footage paired with computer graphics to show body plans and structures too small or too translucent to read clearly on camera otherwise. The series treats each group as a chapter in a larger evolutionary story rather than a catalog of curiosities, tracking how nervous systems, symmetry, and skeletons first appear in these lineages. Locations range across reef systems and open ocean, giving the footage a genuinely global spread rather than a single dive site standing in for the whole ocean. It sets up the seven-hour series that follows, one invertebrate group at a time.