This is the story of the great wildebeest trek.
It began with the birth of a calf.
He was soon faced with many dangers.
The calf even crossed a soda lake.
A variety of spy-cams tracked his journey.
These recorded his adventures in many new ways.
They filmed him surviving an attack by cheetahs.
They even withstood the full force of the rut.
They then staked-out the Grumeti river...
..where Africa's biggest crocodiles stalked the calf.
This was his closest shave so far.
Now, even greater tests await the calf as the herd continues its epic trek.
The Mara River in Kenya, the most northerly point of the migration.
It's late July and the march of the wildebeest stalls
as it faces this fast-flowing barrier.
But there's no turning back.
The wildebeest arrive in tens of thousands,
a unified mob on a frantic mission.
For the adults, it's an annual event.
For the calf, it's the biggest threat so far in his young life.
The crossing points are traditional,
but that doesn't make them any easier,
particularly when the river is in full flood.
Ferocious rapids lie just below the crossing point.
Each year, they claim hundreds of lives.
The calf faces other unfamiliar hazards.
It will be a major test, one that others have recently failed.
A gazelle seems remarkably indifferent to the obvious dangers.
The calf waits but, for the herd,
the gazelle's impulsiveness is a sign that all is well.
As the first ones leap,
it's like pressure being released from a bottle.
Mother and calf wait their turn.
The front-runners leap wildly,
a technique that probably scares the crocodiles waiting in the water.
But the same splashes also sound the dinner-bell to those still asleep.
The one that took the greatest risk paid the price.
Although nervous,
the calf is pressurised by herds massing behind him.
They force him to take the plunge.
At six months old, he is strong enough to tackle most challenges,
but this will test his stamina to the limit.
The rapids lie just downstream.
The teeming throng force the mother and her calf towards the maelstrom.
The pressure builds still more.
The calf stumbles on hidden rocks and loses his footing.
Mother and calf are both swept away by the current.
The whole herd is being washed downstream.
Many retreat, but most are caught in the torrent.
For the calf and his mother, going back is not an option.
Briefly, he finds a foothold...
Then, panic sets in.
Crocodiles deliberately select particular victims.
Mother and calf have no room to manoeuvre.
The mother is grabbed...
The calf is swept away.
The current is so strong that the crocodiles can't fight it.
Only a rock stops their struggles.
The calf, too, is battling for his life.
But the crocodile's hold is awkward.
Reinforcements arrive.
The crocodile is tiring and has to change its grip.
Miraculously, the mother is relatively unharmed.
The calf tries to keep his head above water.
The crocodile is exhausted.
It has to let her go.
Mother and calf rejoin below the rapids.
Together, they make it ashore.
Others are not so lucky.
Every river claims its victims, but each must nonetheless be crossed.
Cheetahs love these open grasslands, too.
There's plenty of space for hunting.
The wildebeest will eat most of the grass before moving on.
The grass reacts by growing.
The more grazing and trampling by the herd,
the more it's stimulated to grow.
But paradise comes at a price.
As well as cheetahs, the migration supports some heavyweight predators.
Lions live here in higher densities than most other places on earth...
And the days of the migration are the best days of their lives.
The wildebeest will stay here for the next three months,
so the threat is something they must learn to live with.
The middle of the herd is the safest place to be.
This cheetah cub is just one week old.
Like all youngsters, play is always on his mind...
But his mother has work to do.
The wildebeest have fellow travellers.
Thousands of gazelle accompany the migration.
Each kind of animal grazes different lengths of grass.
Thompson gazelles eat the shoots left by the wildebeest.
The long red-oat grass provides perfect cover,
but it's too long for the wildebeest to eat.
Given the choice,
wildebeest prefer pastures that have been grazed down by zebra.
The calf runs.
But this time he is not the prime target.
Tortoise-cam investigates.
This apparently harmless creature is of no interest to the cheetah.
The cub is not old enough to eat meat,
but it is risky for him to be alone.
He looks for his mother.
There is plenty to be nervous about.
These are halcyon days for the hyenas, a time of plenty.
A ready-made meal is never far away.
At this age, the calf makes a more challenging proposition.
Hyenas look for signs of weakness.
Despite their reputation, they hunt more than they scavenge.
But when someone else has done the work,
they soon sniff out an easy meal.
Her cub is in mortal danger.
But even for a hyena,
a protective mother is a force to be reckoned with.
He looks for an easier option.
Among the multitudes, there's always one that is sick or injured.
When they reached the Mara,
the population that peaked at one and a half million
on the calving grounds has lost a quarter of a million animals.
Most of those that died were calves.
Black-backed jackals also benefit from the daily cull.
They are expert at snatching food from under the noses of the hunters.
They live on their wits, but are rarely caught.
They also give other scavengers the run-around.
Like a gigantic living lawn-mower, the migration slowly eats its way
across the plains, consuming 6,000 tons of grass a day.
Here the calf will spend a quarter of its year-long journey.
With the herd staying in one place, our spy-cams can be everywhere.
Even a tree stump spies on the calf's every movement.
Boulder-cam, was first used in a film that spied on lions.
Now, it finds a new generation to entertain.
As always, the lions' first reaction is curiosity...
But these cats are more inquisitive than most.
One cub has taken to it in a big way.
Boulder-cam doubles up as a baby-walker.
But then the mood changes.
Boulder-cam captures perhaps the closest shot ever taken
of a stalking lioness.
Every day, the wildebeest go down to drink.
In the Mara, the river provides a source of drinking water
the herds can rely on.
The calf joins the midday throng.
Wildebeest must drink at least every other day to stay in condition.
This is a risky time for the calf.
Nearby, tortoise-cam investigates the remains
of a previous visitor to the river.
The scavengers are experiencing a glut.
Carcasses can remain uneaten for days.
At the river's edge, the biggest risk is from crocodiles.
For the calf, danger can come from an unexpected direction.
This is the perfect ambush situation.
The prey are facing away from the threat.
It's chaos and she's missed her target.
But there are plenty of others to choose from.
They panic onto the plain and, there, other lions wait in ambush.
One is taken.
Then another.
Two kills, within a bound of each other.
Mother and calf keep running.
They can never be sure where the next lions might be hiding.
Two carcasses is just what a large pride needs.
The cubs' births have coincided with this super-abundance of food.
But the Mara is so full of predators
that existing cheek by jowl with other hunters becomes unavoidable.
If they are faced with a large clan of hyenas,
lions often avoid a confrontation.
But these hyenas seem more intent on fighting one another
and that seems to intrigue the lions.
The victim is almost certainly an intruder from another clan.
The lions can't resist making an advance,
but it seems more from curiosity than aggression.
Without further bloodshed, the hyenas call it a day.
The weeks pass,
and the calf is stronger and fitter than he has ever been.
But soon he must leave this grassland paradise and travel south,
back into Tanzania.
He has survived the Mara's minefield of predators unscathed
but soon, he'll have to cross the river again.
His next challenge is quite unexpected.
Hippo-cam is spying on one of the many hippo pods
found along the length of the river.
The hippo seem to accept the strange family member in their midst.
Unfortunately for the hippos, they live at the place
where wildebeest traditionally cross the river.
The herds are already massing on the banks.
Motivated by a common urge, the individual herds have joined forces
to form a giant army, several thousand strong.
For the calf, so much is still unfamiliar.
Hippos are noisy and intimidating,
especially when their youngsters are play-fighting.
Undeterred, the wildebeest push forward.
The mother and calf get their first close look at the problem.
But this blockade must be more than any of them bargained for.
Hippos are notoriously unpredictable
and this pod has a new and boisterous baby.
Even the Thompson gazelles seem more nervous than usual.
For the calf, the excitement seems dangerously infectious.
Luckily, he has second thoughts.
But this was just the encouragement the gazelles needed.
Once they seem confident, the wildebeest are reassured.
The hippos' peace is shattered.
They don't like having their space invaded.
The calf's turn arrives.
He takes the plunge, along with his mother.
The hippos are vanishing fast.
They may be out of sight but they shouldn't be out of mind.
Their mood hasn't improved one bit.
They discharge dung into the water.
Like hidden depth charges, their eruptions break the surface.
The swimmers steer a course around them.
The front-runners reach the shore, pursued by an angry bull hippo.
Another hippo even makes a grab at one of the swimmers.
But it escapes.
In the turmoil, the calf has got himself into trouble.
He's been separated from his mother and is being carried downstream.
The hippo's anger hasn't subsided.
The calf is nearly exhausted but worse is to come.
This is unusual behaviour.
Hippos are supposed to munch grass, not meat.
But the hippo doesn't attack.
Crocodiles aren't so merciful.
But the calf had a good head start.
He may have survived but now he's on his own.
Wildebeest are disappearing away on the opposite bank.
The calf is separated and alone.
There is only one way out and he finds it.
He's back where he started and he has no-one to protect him.
A band of foraging mongooses looks on curiously.
Hating to be alone, lost calves will join any herd they come across.
Others have the same idea.
This impala fawn has also lost her mother.
Like the wildebeest calf, she's desperate for company.
The young impala calls, but the calf ignores her.
But then something remarkable happens -
the meeting of two lost souls.
The calf is curious.
The fawn seems to think he's her mother.
Then, the fawn hears a familiar call - his real mother returns.
This is normal behaviour.
Impala deliberately leave their camouflaged young unattended
when they move away to graze.
Wildebeest calves are conspicuous
and need to stay with the herd to avoid being targeted by predators.
The herd also shares sentry duties.
Alone, it's easy to run into trouble.
This mother cheetah and her large cub are taken by surprise.
The calf takes the prudent option
but one that proves irresistible to the cheetahs.
He can now run like the wind.
The cheetahs are too slow off the mark
to catch up with this 45-mile-an-hour sprint.
Across the river, the wildebeest masses haven't gone far.
The mother returns looking for her lost calf.
She hesitates.
Then she makes a brave decision.
She crosses back to the other side.
Crossing alone has the advantage of surprise.
The crocodiles had little time to prepare.
The cheetahs seem game to chase anything,
even though the mother is far too large for them.
The calf hears her call.
Once more, the two are together.
Another herd appears.
Adult wildebeest show no particular attachment to each other,
so mothers and calves are instantly accepted into any new group.
At one-year-old, the cheetah's cub is able to hunt on her own,
but she still enjoys playing with Mum.
The calf likes his playful moments too.
Play is good practice for hunting.
The mood can change very quickly.
Nearby are the mother and her cub.
The Mara supports a lot of cheetahs!
The others spot them.
The tiny cub is vulnerable
and the pair are acting as if they've seen prey.
The cub is nervous.
Such encounters are rare and it's difficult to predict the outcome.
The mother sounds a warning.
She yelps in distress.
The cub runs for his life.
It's a stand-off as the cub's fate is decided.
Neither seems to know quite what to do.
A reprieve.
It may be that these cheetahs are related, so the competition
from the new arrival is something they can tolerate.
Anyway, the encounter is soon forgotten.
The calf and his mother
have settled back into the herd still left on the Mara.
Migration happens in waves over a period of several weeks.
For those yet to leave, there's still plenty to eat.
Because of the rich pasture,
around half a million wildebeest enter the Mara
at some point on their journey.
Back in the midst of the herd,
the calf relies on safety of numbers for protection.
Predators make daily assaults.
Their days of easy hunting will soon be over.
As many as six lionesses may work together, staging these attacks.
While the grass is long, it's easy to surround the herd.
The lion that has stalked closest initiates the charge.
One misses, but the others still have a chance.
She's got one.
It's a zebra.
Lions seem to prefer them to any other prey.
The productivity of the Mara depends on frequent downpours.
The mother and calf turn their back on the driving rain.
The whole vast herd stands motionless, like statues.
The cub copes with the deluge in his own way.
Although the rain refreshes the grass,
it also triggers mass movement.
The herds are leaving the Mara.
They mass once more on the banks above the river.
They have arrived at the most forbidding crossing point in the Mara.
Opposite them is a sheer vertical wall
and the water below is patrolled by crocodiles.
There are few more difficult crossing points on the river,
but year after year,
the wildebeest choose this place as their final gateway from the Mara.
A gazelle is the first to attempt the challenge.
She reaches the notorious wall
but pays the price.
Zebra are the next to try their luck.
And wherever they go, the wildebeest are quick to follow.
Once the crossing is underway,
the herd's impulse to join in is almost suicidal.
The urge to migrate is so strong, nothing quenches it.
Mother and calf once again wait their turn to join the mayhem.
The mother leaps, the calf follows.
The calf is now a strong swimmer,
but hundreds of wildebeest are putting pressure from behind.
In front stands the sheer 20-foot wall
and still they keep coming.
For the calf, there is no turning back.
The zebra are the first to meet the dangers.
They're the favourite food of crocodiles
but their kick can be crippling.
Mother and calf approach the danger zone.
The crocodile finds a zebra foal instead.
But many more crocodiles lurk beneath the surface.
The calf is singled out.
Missed!
But the danger is not over.
The calf and mother have reached the wall.
The build-up from behind is relentless.
And crocodiles prepare to pick off stragglers.
The calf tries to climb the vertical wall.
Others fall.
He follows.
The crocodile tries again
but the calf is a fast-moving target.
He gradually works his way along the wall.
Yet more wildebeest are arriving.
They use each others' bodies as steps.
And still they keep coming.
The calf is being pushed further along the wall.
Then, suddenly, the pressure eases.
The bottleneck has an opening.
The calf finds it.
The pressure at this one exit is now intense.
The calf and mother make it to the top but the edge is slippery.
The mother tries again, so does the calf.
So near yet so far.
The calf slips.
He struggles to get up.
He finds his feet...
..and manages to reach the top.
His mother is there to greet him.
Mother and calf have survived
some of the most challenging tests in nature.
They will now travel south,
back to the short-grass plains of the Serengeti.
It was here that the calf was born
and it was here that our story began.
And, on these plains, another generation will be born.
They too are destined to make the same epic journey,
one that has been made for a million years.
Undoubtedly, the greatest natural trek on earth.
Subtitles by Red Bee Media Ltd