Replying...
Intro. It takes place in the very first episode of the first season of Doctor's Diary. Gretchen Haase (29) returns from Cologne with her younger sister (24) (the OC). The younger sister is a total high achiever and is currently working on her doctorate. Marc has never met Gretchen's younger sister because, firstly, she was too young to go to school with him, like Gretchen did at the time, and because she was sent to boarding school in Switzerland at an early age. She and Gretchen worked together at Gretchen's now ex-fiancé's practice while she was studying medicine in Cologne. She didn't want to leave Gretchen alone and continue working for Gretchen's ex, so she came with her to Berlin. The scenario is their first day at work, the day Gretchen sees Marc in the elevator for the first time since school. Marc recognizes Gretchen but not her sister standing next to her.

Dr. Marc Olivier Meier

@Mia