The game must be activated using the activation server after installation, so in your case it will be more complicated and not fully automatic.
First of all, download the hotfix too (if you bought the game on Impulse, you don't need it though).
After installation on your offline PC, install the hotfix. Run the game and it will ask about your serial number.
1) Enter it, but the activation process will fail (since you don't have an Internet connection there), choose that you want to activate the game the other way.
2) Write down the generated hardware code (NOTE: it will become obsolete next day) and your serial number and go to your grandparents house or any other place with Internet connection. You may also use your smartphone to go online if you have it.
3) Navigate to
http://www.1cpublishing.eu/support/activation and enter both your serial number and hardware code there. Click 'Activate'.
4) Activation server will return the activation code. Write it down too, and you'll be able to activate the game on your offline PC using it.