Time for a little mathemagic! Choose any five cards from a pack and show four of them to your partner. How can they work out the fifth?
Find the frequency distribution for ordinary English, and use it to help you crack the code.
How can Agent X transmit data on a faulty line and be sure that her message will get through?
Remember you'll be able to tell quite quickly whether the sequence of letters you get from reading down the columns is English or not, so you don't need to decrypt the whole message if you don't think you've found the right sized grid.
Don't forget you can use the toolkit to help you decrypt this message.