One wire in the plug is connected to one side of the generator in the power station. The other wire os connected to the ground. At the power station the other side of the generator is connected to the ground. For a curcuit to be complete the power flows from the side of the generator connected to your plug, throught the plug, through the kettle, through the opther pin in the plug, into the ground and back to the other side of the generator.
If you touch the wire directly connected to one side the generator it flows througn you to ground and back to the other side of the generator.
If you touch the other wire which is connected to ground, although you are connected to one side of the generator you have no connection to the other.
Very simplistic but hope it helps.