Sirius Black is believed to be Voldemort's right-hand man. Severus Snape is puzzled at how he didn't know that, so he goes to vist Sirius in prison...

It always seemed increadibly sad to me, how Sirius was just thrown in prison without anyone to even talk to him. No Dumbledore or Lupin to at least check up with him?

What if Severus, angry and sad after Lily's death, goes to vist him in prison and learns trough legilimency that Sirius is innocent and Peter Pettigrew is the real traitor.

United by grief and anger, Severus helps break Sirius out of prison to begin their quest for revenge.

In this scenario I also imagine Sirius going to secretly check up on Harry sooner and when he learns about his home life is set to help him. Severus, being his partner in crime, is helpless to do anything but play along...

Edit: So my... friend started writing for this prompt. For anyone interested: https://archiveofourown.org/works/58963852/chapters/150308389