The day after Cosette and Marius' wedding, Jean Valjean visits and asks for Marius only. Marius comes to meet him and Jean Valjean spills the beans about his past and about how he's a convict. As you can imagine, Marius is shocked by this. Don't forget that he doesn't know Valjean saved his life, so he's mostly just concerned about Cosette. So he thinks it'd be best if Valjean didn't come around as often anymore.
Oh yeah, and it turns out that Valjean's hand isn't hurt at all. He just pretended it was hurt so he wouldn't have to sign Cosette and Marius' wedding certificate. Had he signed it, the certificate would have been worthless because he would have signed with a fake name.
Around this time, Cosette walks into the room and asks what Valjean and Marius are talking about. Marius tells her that it's private, which makes Cosette pretty annoyed with both of them. Eventually, though, she agrees to leave and does.
Before Valjean leaves, Marius promises to keep this all a secret from Cosette so that she won't have to hear the brutal truth. (Women, you know.)
Once Valjean is gone, Marius can't help but wonder if all the money Valjean gave Cosette has come from immoral practices. Further, Marius also knows that Inspector Javert is dead, and he suspects that Valjean murdered him.