Crypto Algorithm Question - Ciphertext - Encrypt

Hi all,

I wonder if anyone could give me some guide - Given a cipher text which has been encrypted twice with DES, i.e a particular key K was used to encrypt the plaintext and then used again to encrypt the result. And also given five bytes of the eight-byte key. I want to find out - How we can find the full key and the plaintext?

Can anyone assist me on this.
Thanks in advance.
Sam