What kind of plastic was it? just a piece of saran wrap or was it from something else? I have scrubbed off melted plastic wrap and a small bit of melted end from a turner and had no ill effects but cant say there is no risk from chemical residue that might be left over.
You would need to re-season after cleaning but you can use steel wool or a wire brush to scrub it off. I would then use salt and baking soda and a little water to make a paste and scrub some more to be sure its really all out of there. Then once you believe it is all off rinse well and put it in the oven and run a self-clean cycle or bake it at 450+ for a half hour then turn off oven and let cool in there overnight. Once that is all done wash with water and dish soap then rense well and dry then re-season. It will take some uses before the scrubbed area begins to really be seasoned well so watch for sticking.
it depends if the pan looks like it was well maintained and has a good black patina(season) I would just wash with hot water mixed with bleach to kill any lingering bugs and a plastic scrub pad (scotch bright) so as to not damage the season too badly. if on the other hand the seasoning is quesionable or there is any rust at all on the inside I would use the extra method using wire brush + salt and soda just to make sure its really clean and then re-season from scratch. I am pretty sure baking it at 450F for 1 hour will killl any possible pathogins.
found this on wiki so not sure how accurate it is:
Dry heat can be used to sterilize items, but as the heat takes much longer to be transferred to the organism, both the time and the temperature must usually be increased, unless forced ventilation of the hot air is used. The standard setting for a hot air oven is at least two hours at 160 °C (320 °F)
If there is a lot of rust/crud/corrosion you can take it to a local auto paint shop and ask them to beadblast or sandblast it down to bare metal and season from scratch they will usually do it for free or just a couple dollars. I have a huge (15-20 Qt?) dutch oven probably from the 19th century or very early 20th I dug out of an old barn that was rusting (only surface rust on the outside) and had been the home for some mice for some years so to be sure i got any toxins out I had it beadblasted for $10 and re-seasoned it, only use it when at our vacation cabin after maybe 10 uses its almost completely black again and cooks a dream making stews/chili in the fireplace.