Moskva question

Just read the book, thoroughly enjoyed it. I had to concentrate and believe I understood most of it, but it was still complicated and can't find anything [from Googling] about the 'ending explained' just to tie everything up in my own mind. PLEASE would someone concisely explain the ending with the characters? E.g. did Dennisov betray Tom Fox or was that the Grandfather of the young Dennisov, bit confused!?