Of course, as with most budget airline promotions like this, there's some important fine print to consider. For one, these super-cheap fares are available within a fluctuating travel window based on departure city, but the bulk of the best priced flights are only available for trips between now and early 2018. Also, $99 is just the one-way ticket price, meaning if you intend to return you'll be paying significantly more than that for a round-trip ticket (as of this writing the cheapest return ticket was $119). Norwegian also charges a la carte for meals and seating assignments, but does allow you a free carry-on and personal item (so, save yourself some money and pack like a pro).
If you need some time to mull over where exactly to high-tail it to across the pond, you're in luck because this particular sale runs until October 2 at 11:59pm.