The first series, "The Nightmare Begins", is a set of 25 stories about a teenage boy arriving at the hotel after being lost in the woods and taking a taxi to get back to home. He suddenly encounters the mysterious owner of the hotel, an old rat known as Gregory, who suggests that he stay awhile in Room 205. The boy soon becomes drawn into a bizarre series of events taking place within Gregory House as he tries desperately to escape. Though he eventually escapes back to his friends and family, he soon discovers that none of them remember him. In the post-credits scene, a mysterious man approaches Gregory House and asks Gregory if his son was here.

The second series, "The Second Guest", is similarly set with 25 stories, except this time the main victim is a teenage girl who has just taken a taxi home from her best friend's school ceremony. She has had to endure many failed relationships and being isolated. It is also implied that her old flame was once the first guest, but their relationship went disastrously wrong. Near the end, she can finally escapes Gregory House. Due to Neko Zombie's increasingly erratic mental state, he burns down the house - however, Gregory and James survive, while Neko Zombie and Gregory's Mom will stay dead forever.

The third series, "The Last Train", is set with 26 stories during a bizarre train ride that Gregory embarks upon. It differs from the previous two series in that there is no guest, nor is the hotel the central setting. Instead, Gregory himself is the protagonist, and the series takes place on the previously mentioned train as Gregory tries to escape from his own home and his role as the innkeeper. However, unlike the previous two protagonists, he fails, and the train only takes him back to Gregory House.

The fourth series, "The Bloody Karte", is a set of 12 stories about Catherine working in a mysterious hospital.

Source: Wiki