Naruto might initially be angry and shout for someone to let him out. He has a strong will and doesn't like being confined. But as time goes on, he could become more creative. He might start to communicate with his friends secretly through some ninja means, like sending messages via small animals. He would also likely be thinking about how to prove his innocence if that's the reason for his confinement. He's not the type to just sit around and accept an unjust situation.
Maybe he's being punished for some misdeed in the fanfiction world. For example, if he accidentally broke an important village rule or caused some chaos during a mission. Another reason could be that someone is trying to keep him safe from an outside threat. They might think that locking him in his house is the best way to protect him from enemies who are after him.
At first, Naruto might be confused and hurt. He doesn't understand why his father would do such a thing to him. He may feel betrayed since he expects his father to be a source of love and protection, not confinement.
In this fanfiction, it's possible that Naruto is locked away as a result of a misunderstanding. His powers might be seen as uncontrollable by some in the village. They fear what he could do accidentally with his strength, so they lock him away in an attempt to keep the village safe. However, Naruto being Naruto, he would surely find a way to prove his innocence or break free in the end.
It could be the village council. They often make decisions for the safety of the village, and if they perceive Naruto as a threat, they might be the ones to lock him away.
The house might hold the truth about Naruto's birth. There could be records or something that reveals more about the circumstances of his birth, like why he was chosen to be the Jinchuriki of the Nine - Tails. This would be a huge revelation for Naruto.
One possible reason could be for Naruto's protection. His father might think that by locking him away, he can keep Naruto safe from the many threats in the Naruto world, like powerful enemies who might target Naruto for his special powers.
One possible plot could be that Naruto is locked in a cage as a result of some dark jutsu used against him by an unknown enemy. His friends then have to embark on a journey to find the key or the counter - jutsu to free him. Maybe they face many trials along the way, and Naruto, while locked, has to struggle with his inner demons as well as the physical confinement.
He could uncover the truth about his father's real mission before he died. Maybe it was something that was kept hidden from the village for some reason.
In this fanfiction, Naruto might find some long - hidden family secrets in his parents' house. Maybe he discovers old ninja tools that belonged to his father, which could give him new powers or insights into his family's past ninja techniques.