Freddie and the Two Mystery Cats

This is a story about a lady called Millie who lives in her white cottage with her cat Freddie. Freddie is a house cat, until one day he wants to explore the neighbourhood. Instead, when it's time for dinner he doesn't come but a mystery cat comes in his place. The same happens the next day and Millie doesn't know the difference between the three cats. The question is, who are these mystery cats showing up at Millie's door?