Leo gets bitten by a parasitic mutant bug, and watches over its egg. But he keeps watch over it for days. His brothers start to worry, and when Raph fights his infected brother, Leo bites him, Raph gets infected as well. Mikey and Donnie fight them, and Raph bites Donnie. It's now up to Mikey to finish the antidote Donnie started and save his brothers.