The chlorine or chloramine must be removed prior to mashing or mixing in the malt extract, depending on the way you make beer. It reacts with phenolic compounds in the wort to give you that medicinal plasticy flavor and aroma. By the time you smell it, it's too late to do anything about it.
Chlorine will boil out, chloramine not so much. Has to be done before using the water in brewing, as the chlorine reacts faster with the phenolics than it boils away.
One campden tablet for 20 gallons of water, five minute contact time, and your good to go, but do it before you use the water to brew with.
The reason you can't do anything after the fact is because the chlorine reacts with the phenolics to produce a chlorophenols. These are chemically very stable and the SO2 from the campden tablet won't remove the chlorine once it's bonded to the phenol.
Reason: added information