Ends. Then book a cheap flight from the city the cruise ends in back to the city it started at. Flights around Europe are cheap on carriers like Ryanair.
You’ll get an extra day in the city the cruise ends in and the cost of the one way trip back to catch the return leg of your round trip will be a fraction of the cost of 2 one way tickets.
I’ve done Ryanair several times in Europe and flights we usually just $50-70 for a one way ticket.