Originally posted by: TastesLikeChicken
Knock it into the P-trap and then unscrew the P-trap compressing fittings (assuming your P-trap isn't soldered) and dump it out. It's probably easier than trying to fish it out.
It should be easy to undo the P-trap and dump the content out, and some P-trap are glue type that may have a drain/clean out at the bottom.
The cap shouldn't be a problem on your drain, unless you have a partial clog some where down the line. The water pressure from a full basin should be more than adequate at flushing the cap out off the building drain, and run the cold water tap on full for a few minutes should flush the cap out off the building sewer into the city sewer.