Ep 3. School Reunion

When the Doctor and Rose crash-land in present-day London, they get an unexpected education at a school reunion. For years, the Doctor has had a nagging feeling that he's forgotten something important – something that has to do with a former companion, Sarah Jane Smith. He discovers that Sarah Jane has also been investigating strange activity at a local school, where she's posing as a dinner lady. The Doctor is overjoyed to be reunited with Sarah Jane, but he discovers that her life has been very different since they last saw each other – she's now a journalist, widowed, and a mother. Meanwhile, Rose is feeling jealous of Sarah Jane's close relationship with the Doctor.

As the trio investigate the school, they discover that the headteacher, Mr Finch, is using alien technology to improve the students' academic performance in exchange for the students' minds. They also encounter K9, Sarah Jane's robot dog, who gives them much-needed assistance. The Doctor, Sarah Jane, Rose, and K9 hatch a plan to defeat the alien threat.

Despite the success of the plan, the Doctor is forced to acknowledge that his former companions age and move on without him. Sarah Jane confronts him about leaving her behind, but ultimately forgives him for the pain he caused her. Rose, too, must come to terms with the fact that the Doctor has had other companions before her.

This emotional episode addresses the themes of aging, loss, and the bittersweet nature of memories. David Tennant's charming portrayal of the Doctor is complemented by Elisabeth Sladen's return as Sarah Jane Smith, a beloved character from the classic series. School Reunion is a poignant reminder of the effects of time on relationships, and how we must cherish our memories while acknowledging that they can never be recaptured.