In this chapter, Joseph deceives his brothers again by planting evidence to incriminate Benjamin of theft.
Joseph fills their sacks with money for a second time, but also adds a special silver cup into Benjamin's sack. After commanding his steward to do this, he then commands his steward to go chase them down and arrest Benjamin for theft.
Verse 9 and 10 show that, as far as I can tell, Joseph's plan was to bring Benjamin to Egypt and then keep him there. He manipulated his brothers last time to get them to bring Benjamin down, and now he is manipulating them to keep Benjamin there. He gave his brother Benjamin "five times" the portion of any other in the last chapter, as well as blessing him and weeping at his sight. So it's pretty evident that he doesn't intend to harm Benjamin at all, and from his unnamed steward's words in verse 10, we see that he plans on letting all of his other brothers go back to Jacob.
The fundamental tension here is that Jacob also loves Benjamin, as he made clear last chapter. So for Joseph to have Benjamin would bereave his father. Judah also swore to his father to return with Benjamin, and hence Judah now has his second noble act in the Bible, standing up to Joseph and pleading to exchange his life for Benjamin's.
Lastly, it's interesting that this chapter says that Joseph uses his silver cup for divination, and he says "Do you not know that a man such as I can practice divination?" This is interesting because divination is specifically outlawed by the Law of Moses, yet here Joseph practices with no explicit condemnation. This can probably be ascribed to his many years in Egypt, serving Pharaoh, where divination would have been common. It's a pretty brief reference so I don't want to make too big a deal out of it, but I always find it notable when characters who are otherwise positively regarded do things in contravention of the Law. Joseph is just another one of those characters.
Judah takes a long and humble stand that he has to exchange his life for Benjamin's, for the sake of his father, and as we will see in the next chapter, Judah's words have enough emotional force that they cause Joseph to reveal himself, rather than continue with the charade.