Michael A— lives a quiet, comfortable life since his retirement from the intelligence services. Practically a recluse, he spends his days imagining the lives of the anonymous people he watches in the park beneath the window of his elegant New York townhouse—number 221—his every need tended to by his housekeeper, Mrs. Baker.

For weeks, a girl has sat in the park every morning at dawn. Always alone. Always watchful. And when the sun rises, she vanishes, as if she was never there.

But one day her routine changes—and Michael realizes that she faces terrible danger. He makes an uncharacteristic decision to abandon his solitude and help her. Soon, Michael finds himself plunged into the New York underworld, and he’ll have to use all the tricks of his former trade if he’s to keep not just himself, but his new friend, alive.