Erin Allin O'Reilly

The socially stunted guys are nervous when Penny invites them to a party she's hosting.

Sheldon gets sick and Penny has to deal with him.

After returning to her job at the Department of Defense, McGee's girlfriend Delilah finds a break in a controversial case, prompting her to turn to the NCIS team for help. Meanwhile, McGee talks to DiNozzo about whether he should ask Delilah to move in. Margo Harshman guest stars.

It's Richard's birthday and Bailey invites him to scrub in on the case of a lifetime. Meredith hires a research assistant to help with her portal vein project, and a scorned Callie gives Derek quite the shock.

Sister Mary Eunice is terrified to discover a dark angel has descended on Briarcliff. Kit makes a bold move to be reunited with Grace.

The NCIS team consults with three cyber-terrorists they imprisoned after a city-wide Internet shutdown the week before Christmas. Also, McGee reflects on the case and Gibbs' infamous rules as he pens a heartfelt letter to his father.