Grey's Anatomy celebrated Valentine's Day last night, while Private Practice introduced viewers to Sam's troubled sister.

Next Thursday, though, both these Shonda Rhimes creations will come together once again, as the latest crossover event will feature Amelia arriving at Seattle Grace to beg her brother to help save Erica's life. SPOILER ALERT: Derek accepts the mission.

Grey's fans can also look forward to Cristina and Owen undergoing marriage counseling, while the residents continue to study for their boards; and Private Practice viewers should expect Scott to try and take his relationship with Violet to the next level.

Watch a combined promo for this two-hour event now:

Matt Richenthal is the Editor in Chief of TV Fanatic. Follow him on Twitter and on Google+.