Mike's box is one of the many objects seen on Desperate Housewives that serves of big importance in the unfolding of a story.
When sorting through Mike's things with the intent of giving them up for charity, Lee picks up a light black box, which is locked, and asks Susan what it is. Susan says she doesn't know, as she's never seen it before, and says she'll grab Mike's keys. Lee tells her maybe her opening it is not a good idea, and she asks why not. Lee says "Because it's in your house, in your bedroom, but it's locked. So maybe Mike didn't want you to know what was in it." Susan says that's ridiculous, and adds it's probably just papers and stuff inside the box, and Lee says she's probably right, but if it's something else she might not want to know. Susan asks what he suggests she do, and he suggests that she go and make herself a nice cup of tea while he opens it, and if it's just papers he'll tell her. If it's something else he will decide if she should see it. Susan isn't too keen on the idea, but acquiesces.
Lee is reluctant to show Susan the contents of Mike's box.
Later, when Lee comes down the stairs with the box, Susan stops him, asking about the box. Lee says he actually couldn't open it, and then he tries to rush off, but Susan stops him, asking if it is that bad. He begs her to listen to him: Mike is not defined by what is or isn't inside that box. Susan says she has a right to know, as she's his wife, and Lee gets it, but he's just trying to protect her.
However, some time later, curiosity gets the best of Susan and she begs Lee to show her the contents, which he does. Later, they are sat at a table, with the box open. Lee takes out his passport, letters from his mom, a cute picture of Susan and MJ... and a cheque book. Apparently Mike set up an account eight years before, when he and Susan were divorced. Lee tells her every month since then he's written a check to some woman named Jennie Hernandez. Susan says she's never heard of her. She starts to grow concerned.
Susan is shown the drawings inside Mike's box.
Lee then takes out a piece of paper and says it's Jennie's address. It's somewhere in Mount Pleasant. Susan notices something else inside the box and takes it out. Drawings. One of a field of flowers, and the other one of a butterfly. Susan is surprised, and Lee asks if there is any chance those drawings were made by MJ. Susan denies this, and when Lee tries to come up with some hypothetical explanation, Susan stops him, as they both know what this is... "There's a child out there. Mike's child."
Later it is revealed the drawings were actually made by Mike's autistic sister, Laura Delfino, and the cheques were for her. ("With So Little to Be Sure Of")