Liz Bonnin presents a controversial and provocative episode of Horizon, investigating how new scientific research is raising hard questions about zoos - the film explores how and why zoos keep animals, and whether they need to change to keep up with modern science or ultimately be consigned to history.
Should zoos cull their animals to manage populations? Liz travels to Copenhagen Zoo, who killed a giraffe and fed it to the lions, to witness their culling process first hand. They think it is a natural part of zoo keeping that is often swept under the carpet. Should some animals never be kept in captivity? In a world exclusive, Liz visits SeaWorld in Florida and asks if captivity drove one of their orcas to kill his trainer.
But could zoos be the answer to conserving endangered species? Liz examines their record, from helping breed pandas for the wild to efforts to save the rhinos. She meets one of the last surviving northern white rhinos and discovers the future of this species now lies in a multimillion-dollar programme to engineer them from stem cells. Veteran conservation scientist Dr Sarah Bexell tells Liz the science of captive breeding is giving humanity false hope.

取材于抗日战争时期中国军民协力以军犬为武器抗击日本侵略者的史实。地下党员关汉良为寻访良犬乔装来到武平山中村落。关与日军官八木皆发现二林的狗海龙是罕见的优秀狼犬。海龙为救主人不幸身亡，其幼犬大龙和小龙加入了关的军犬突击队。经过一年的强化训练，它们成为了军犬队的头犬。八木率军疯狂进攻游击队及附近的村庄，大龙赶回军犬训练基地后牺牲，小龙将八木引至海龙的墓前，以自己的方式报了仇。