When Did Saint Patrick Die?

The story

Journalist Mitchell Brown meets a tipsy woman at a bar and takes her to his house to spend the night because she is very drunk and cannot control herself. He looks for her documents, but finds only a checkbook with a strange number. The next morning the stranger disappears. Some time later, in the same bar, Mitchell meets her again in the company of a man who turns out to be her husband. She pretends not to recognize Mitchell. It turns out that there was a murder in the city that evening and she needs an alibi.