When They Last Saw Each Other: The Lion and the Rose (S4E2)
What Happened Then: After being forced to marry each other, Sansa fled from the Purple Wedding while Joffrey was dying, with Tyrion getting the blame for his murder
What Happens Now: Sansa needles Tyrion in this episode, while it's clear how much respect he has for her. They're on the same side though, for now at least.