The film is just a series of split-second fortuitous events and lucky close-calls that lead Jackson Curtis and his family from airport to airport, smaller plane to bigger one and towards their destination - China.
In Vegas they hitch a ride with Jackson's limo client, the wealthy Russian businessman Yuri, on board a giant Russian cargo ship (carrying his luxury cars to a US roadshow). Gordon is co-pilot to Sasha (the best character in the movie), and they clear the runway just as the ground beneath them crumbles - and on the way up strike off against the Vegas Eiffel Tower. They aim to refuel in Hawaii - but the whole island is on fire, so plan B is to crash land in the South China sea, 1500km from their destination.
Fortunately with the polar shift, the sea is now a mountainous snow range. Without working landing gear they crash land in the snow, and use the luxury cars to cushion their escape from the fall. Sasha remains. The plane stops at the edge of a cliff. He breathes a sigh of relief. Then it topples forward, falls a few thousand feet and explodes.
The Chinese army are flying overhead - they stop to pick up the Russian and his twin boys (as they have green passes - each worth 1billion Euros), leaving his girlfriend (and her dog) and the Curtis family. But they're ok as they manage to hitch a ride with a Chinese family passing by.
Meanwhile, the spaceships are in fact 12 giant metal Arks - each holding 400,000 people (or was it 400,000 total?), with a different race or group of countries in each. Plus the odd giraffe. There is a problem with the Russian ark - and Yuri is denied entrance. Pretty harsh seeing as though he spent 3billion Euros to fit them out.
The Chinese family and Curtis make it to a back gate. The welder son who has been working on the ships is reluctant to let them through, but Kate gives the "I'm a mother" speech to the old woman, and she makes her son do the right thing. They start to climb up through different mechanisms to get in through a hidden secret doorway.
The scientist who announced the disaster in the first place is amazed that other arks are not willing to help all the thousands of people still in the factory. He appeals to each ark's captain, and they agree to open the gates.
It's mayhem. Yuri and his boys are pushed over the edge, but manage to hang on.
This causes new problems for the Curtis family as the gears start to trap them. The Chinese son is maimed, Gordon is squashed, and their rope is caught up in a wheel and jams open the ark's doors.
Yuri gets back to the walkway, and throws his sons over the rising doors - but in doing so falls to his death.
The American ark's people start to panic. The tsunami waves strike earlier than planned, and the ark with all the heroes on board is pushed from its supports. It is heading straight towards Everest - and without the doors shut the engines won't work to move it to safety.
The scientist spies Jackson on the CCTV and asks his friend to be a hero. Jackson swims off and, helped by his son, frees the cable just in time. The gates shut, engines start ... but the ark still crashes into the north ridge. The damage wasn't too severe and they manage to pull away. They set sail for pastures new with a few other, friendly, arks alongside.
The title-over shows that it is D-Day + 27. Satellite imagery show that most of mainland Africa was unaffected by the floods, so they set sail for the Cape of Good Hope. With pollution and volcanic ash dying down, the metal windows are opened and people are allowed on the top deck for martinis and photographs. Jackson is back with his family, while the scientist is getting friendly with the President's daughter. He died by the way in all the carnage before.
The alternate ending is that Adrian the scientist receives a call as they are heading towards Africa from his father. He is understandably overjoyed to hear that he is still alive - but so is everyone else it seems on the cruise liner - even old Tony who just has a broken wrist to show for the "A perfect storm" type waves before. As the camera zooms out we see the cargo ship stranded on a rock, with the arks closing in ... to finish the job. No, just kidding. But that would have been a good twist Mad-Max style!