The episode summary from tv.com:
"While working to locate a missing local girl, Allison learns about secrets from Cynthia Keener's past. Meanwhile, Joe's relationship with Allison is tested after Bridgette's science class project sparks his imagination."
Sarah will be playing the missing local girl Susan Keener.
You can find the trailer for this episode here. I also screencapped it and those caps can be found here. Here's a nice teaser pic:
This show airs at 10pm eastern and 9pm central time. This is also a two parter so the second half will air next Monday March 31st at the same time.
In other Sarah news:
This Fall the WB is going to make a comeback online and you can see old episodes of Everwood for free as well as many other WB shows.
More info can be found here.