Joy and Mark investigate the reproductive secrets of marine mammals to learn how animals that breathe air manage to mate, give birth and raise their young underwater. Joy travels to New Zealand to uncover the mating strategy of dusky dolphins. While she reveals the adaptations marine mammals have evolved to mate in the ocean, Mark travels to Mexico to find out how whales find mates in the ocean vastness. Witness a dolphin birth, explore the difficulties of breastfeeding underwater and learn how dolphin and whale young avoid predatory killer whales.