Sir David Attenborough turns his attention to the creatures living on bustling coral reefs.
One of our favourite sequences sees a turtle having a spa day, lying back blissfully while smaller fish kindly remove its dead skin and parasites.
Meanwhile, a cute saddleback clownfish has to drag a coconut shell along the seabed to give his mate a safe place to lay her eggs.
Competition for food is fierce on the reef and, while a broadclub cuttlefish changes its skin pattern repeatedly to hypnotise its prey, a canny coral grouper teams up with an octopus to nab a meal.
More worryingly, the episode also looks at how rising temperatures are causing an increase in coral bleaching.