DIAGNOSIS That plastic taste usually means one of two things: either the new filter wasn't primed correctly and you're tasting the carbon media, or there's a biofilm buildup inside the pitcher's reservoir that the filter isn't catching.
FIX Let's start with the filter. A new one needs a solid 15-minute soak in cold water before its first use. If you didn't do that, take it out, soak it in a bowl of cold water for 15-20 minutes, then reinstall it. Next, give the entire pitcher a deep clean. Dump any water, disassemble it completely (lid, reservoir, pitcher), and wash every piece with mild dish soap and warm water. Use a soft bottle brush on the reservoir. Rinse everything extremely thoroughly.
IF THAT DIDN'T WORK Check your source water. If you're filtering very hot water or water with a high chlorine content (like from a well), it can sometimes react with the materials. Always start with cold tap water. Also, make sure you're not storing the pitcher in direct sunlight or near heat sources like the stove, as that can accelerate plastic leaching.
HEADS UP The plastic taste can sometimes linger for a full pitcher or two after a proper filter prime and clean. Run a couple of full cycles through and discard that water before you drink from it. If the taste persists beyond that, there might be a defect with the specific filter.