Linking cells values across different Documents or Spreadsheets

I have been trying something but have been unable to find a viable solution.

I have a table in which the values in two columns are interdependent, i.e. if there is 'x' value in the first cell, there has to be 'y' value in the corresponding cell in the other column.
I also have a separate spreadsheet 'S' listing all the x-y combinations.

Now i want that if I write 'x' in a cell in column-1, the corresponding cell in column-2 should automatically display 'y', taking values from the separate spreadsheet 'S'.

Can anyone please help me with this...
Ask me to elaborate if you cannot understand.