The migration of wildebeest is one of the most familiar events on the globe.
Each year, over a million wildebeest and many hundreds of thousands of gazelles and zebras gather in one corner of Tanzania's Serengeti plains.
And for the predators here, with this plentiful food supply, it is a time to feast.
But what happens when the wildebeest and other grazers have gone?
Over the course of seven months, the BBC Nature's Great Events team followed a pride of lions, filming what happened to them in this land of constant change.
And the story was a harrowing one.
Everyone thinks of the lion as the "king of the beasts" - but when the wildebeest leave, these big cats do not follow.
They have a life system that is based on having home ranges and they hunt within this territory.
When they have thousands and thousands of wildebeest there, that is their time of paradise. But when the wildebeest leave, everything changes - this great larder of food just walks away.
The year that we filmed was particularly hard for some of the lions - and for our cameraman Owen Newman, who has spent the past 20 years filming big cats in Africa, it soon became clear to him that the Ndutu pride, with its seven cubs, was struggling.
As the grazers left, the pride's four females struggled to find enough food and water, and over a period of just a few months, the number of cubs dwindled from seven to just four.
Even with all of his years of experience, Owen found this extremely difficult - he had never seen cubs so thin before, and the lions' plight began to affect him deeply.
And when two of the four cubs - a little female and its brother - became separated from their mother, calling out, emaciated and alone, Owen was sure that they were close to the end.
It seemed almost disrespectful to capture what could be their final hours on camera, so he finished filming as soon as he could and left them alone.
The next day, when he returned to the same spot, they had gone - and he assumed the cubs had died.
Unable to shift the scene from his mind, Owen kept on watching for the cubs - and to his surprise, one of them, the small female, who had seemed so close to death, managed somehow to rediscover the pride.
But just as this reunion provided a glimmer of hope for these desperate lions, the pride vanished and Owen was unable to find them before he had to return to the UK.
Months later, Owen returned to the Serengeti to resume filming.
The wet season had begun and the wildebeest were arriving and giving birth - but he could not shift the pride that had suffered such hardship from his thoughts.
He and his team were constantly on the look-out for them, using photographs of the lions' unique whisker patterns to see if any of the cats that had returned to feast on the wildebeest matched the animals he had filmed a few months before.
Finally, a local spotter reported a sighting.
And, when Owen reached the scene, he discovered that the pride had survived and the last two cubs were still alive - the little lioness that had rediscovered the pride and who had been so close to death's door just a few months earlier was now healthy and well fed.
For all of us, and especially Owen, it was an emotional and moving experience, and the little known story of what happens to the lions when the wildebeest are gone will surprise many people.
Nature's Great Events: The Great Migration is on Wednesday 25th February on BBC One at 2100 GMT and is repeated on Sunday at 1800 GMT
Learn more about big cats and Big Cat Rescue at http://www.bigcatrescue.org