How did it happen in the movie?
When Caradhras bars the road through the Misty Mountains, the Fellowship is forced to take the route underneath the Mountain: through the Dwarf-city of Moria.
Small problems arise when the company arrives at the West Gate: Bill the Pony has to be dumped, Gandalf can’t seem to get the doors to work (though Frodo quickly saves the day) and they are attacked by the Giant Squid from Hell.
Once inside, they find that all the Dwarves are dead and it is very very dark inside the mines. Halfway through the journey the Fellowship is attacked by Orcs after Pippin accidentally pushes a skeleton into a well in Balin’s Tomb.
They flee across the great stairs to the East Gate, but are pursued by a Balrog who – despite being defeated by the old wizard – drags Gandalf with him in his fall.
Outside, Aragorn fears another action-packed scene with Orcs hunting our brave heroes and doesn’t allow the companions much time to grieve the passing of their leader.

How did it happen in the book?
The events in the Moria-sequence are described in three different chapters, but we chose to chop this long sequence into four chunks:

The Gates of Moria compares the scene outside the doors with the material offered at the beginning of 2.IV. A Journey in the Dark. The events in the same chapter reoccur in A Journey In the Dark which deals with the scenes in the movie where the Fellowship travels through Moria.
Balin’s Tomb and The Bridge of Khazad-dûm compare these two scenes with the events described in 2.V. The Bridge of Khazad-dûm.