This is a question we come across often. Electroplating involves depositing one metal which is connected to the anode on to another metal which is connected to the cathode.
Non-metals such as glass, plastic can be electroplated after they have been coated with a conductive paint. The conductive paint provides a surface on which the coating can take place. Another method of providing a conductive surface is the use of amorphous carbon powder over which the coating can be done.
More recent techniques involve etching the plastic surface with Chromic acid. The acid is then neutralized. The surface of the plastic is activated by with a solution containing tin or palladium. The surface is then coated with a layer of nickel or copper.