莉兹·波宁（Liz Bonnin）展示了一个具争议性和挑衅性的《地平线》，探讨了新科学研究如何引发有关动物园的棘手问题-电影探讨了动物园如何以及为什么饲养动物，以及他们是否需要改变以适应现代科学或最终被委托历史。
动物园应该宰杀动物来管理种群吗？利兹（Liz）前往哥本哈根动物园（Copenhagen Zoo），后者杀死了一只长颈鹿，并将其喂给狮子，亲眼目睹了它们的淘汰过程。他们认为这是动物园饲养的自然组成部分，经常被地毯遮盖住。某些动物永远不要被囚禁吗？在一次世界独家比赛中，利兹（Liz）参观了佛罗里达的海洋世界，并问囚禁是否驱赶他们的一个逆戟鲸杀死他的教练。
但是动物园能否成为保护濒临灭绝物种的答案？利兹检查了它们的记录，从帮助野生大熊猫到拯救犀牛的努力。她遇到了最后一个幸存的北部白犀牛之一，并发现该物种的未来现在处于一项耗资数百万美元的计划中，以利用干细胞对其进行改造。资深的保护科学家Sarah Bexell博士告诉Liz，圈养繁殖的科学给人类带来了虚假的希望。

  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.