The fifth programme explores the alliances formed between the animal and plant worlds. Attenborough dives into Australia's Great Barrier Reef and contrasts the nocturnal feeding of coral, on microscopic creatures, with its daytime diet of algae. Some acacias are protected by ants, which will defend their refuge from any predator. Besides accommodation, the guards are rewarded with nectar and, from certain species, protein for their larvae as well. Fungi feed on plants but can also provide essential nutriment to saplings (Mycorrhiza). The connection is never broken throughout a tree's life and a quarter of the sugars and starches produced in its leaves is channelled back to its fungal partners. Meanwhile, fungi that feed on dead wood leave a hollow trunk, which also benefits the tree. Orchids enjoy a similar affiliation. Lichens are the product of a relationship between fungi and a photosynthetic associate, usually algae. They are extremely slow-growing, and a graveyard is the perfect location to discover their exact longevity. Mistletoe is a hemiparasite that obtains its moisture from a host tree, while using own leaves to manufacture food. Its seeds are deposited on another by the mistletoe tyrannulet, following digestion of the fruit. The dodder (Cuscuta) is also parasitic, generally favouring nettles, and siphons its nourishment through periodic 'plugs' along its stem. The rafflesia has no stem or leaves and only emerges from its host in order to bloom — and it produces the largest single flower: one metre across.

On a dusty highway between Australia's most isolated city and its largest gold pit lies Coolgardie - where the arrival every three months of a new pair of foreign backpackers to work the only bar in town is a keenly anticipated event. Fresh off the plane and lured by the promise of an authentic outback experience, Finnish travellers Lina and Steph find themselves en route to a dot on the map - to pour beers, replenish depleted travel funds, and live amongst the locals. But their working holiday quickly deteriorates into a baptism of fire.
Harangued by their new boss, relentlessly pursued by booze-addled patrons, and prey to the madness and malaise of an environment as claustrophobic as it is isolated - the girls soon realise that to meet expectations out here, they'll need to do more than just serve drinks.