As we might have guessed (but were never told) House has been seeing Dr Nolan regularly since he left the sanatorium. The episode begins with House in a therapy session with Nolan, telling him about his week. Wilson has asked him to move out since he and Sam are now together. He also talks about his newest patient, who is a woman with amnesia. House is not in a good state.
He goes to check on his old apartment, and finds Alvie has been living there without his knowledge. He is surprised to see him, but in the end is happy that he is there, for he is a substitute for Wilson in House's loneliness. Wilson visits House and finds him with Alvie. House focuses his attention on Alvie, and pays relatively little attention to Wilson, as if trying to show him that he does not need him.
We learn that House has been pawning some of his possessions: his finances are apparently not in great order. Now however, House wants these back, but when he goes to the pawn-shop he finds that some of them have already been sold, especially a book he apparently puts some special value on.
Wilson discovers this, and finds out the address of the person who bought the book. He gives this to House, who tries to buy the book back, but without success. Alvie, when he learns of this, goes to the address and steals the book for House.
Apparently House has been involved in disturbing things, unbeknownst to his friends; in particular, he was involved in a fight in a bar while he was drunk. Nolan discovers this, and apparently tells Wilson, who apparently tells Cuddy, for she pages him to come to her office, ostensibly to talk about the patient. When he gets there he discovers Wilson there as well. The details of what happens there are not clear, but it is almost certainly a discussion of his behavior, and is an attempt by his friends to help him. It looks like he rejects these efforts.
Wilson is very worried about House, and after he discusses it with Sam, he asks House to come back to live in his apartment. House predictably refuses.
House now asks Alvie to leave: he tells him he wants to be by himself. The last scene shows him in a therapy session with Nolan. When he leaves, he makes clear that he is not going to come back.
As I mentioned, Cuddy is still with Lucas in this episode. She gets only 2 scenes.