The episode begins at a modern day comprehensive school, with the headmaster, Mr. Finch, questioning a student as to why she is sitting outside his office. She claims she has a headache, and that she has not gone home because she lives at the orphanage. Mr Finch tells her to come into his office, at which point a scream is heard and a mysterious shadow is seen.

The Doctor then appears, undercover, as the replacement teacher of a science class. He asks the class a few simple questions about physics, which no-one can answer except a student called Milo. After trying out a few more questions, the doctor advances onwards through higher levels of knowledge, finally asking how to travel faster than the speed of light. Milo answers this, suprising The Doctor.

As the Doctor goes to eat lunch in the cafeteria, Rose is shown, working undercover as a dinnerlady. She comes over and talks to the Doctor. The Doctor comments on odd-tasting chips, then the headmaster, Mr Finch can be seen encouraging other students to eat theirs.

Mr Wagner tells a girl named Melissa that she has been moved to top set and that Milo has failed him. Melissa goes with Wagner to the maths room.

Meanwhile back in the kitchen, the dinnerladies are moving in a large barrel. The head dinnerlady does not want one drop spilled. Rose phones Mickey to tell him about the barrel. Then it tips over onto a dinnerlady and starts to burn her. The dinnerladies move her into a room where a scream and lots of smoke appear. Rose is confused.

In the staffroom, The Doctor is talking to Mr Parsons, the Head of History. Parsons says that when Finch came as head 7 teachers came down with flu and they were replaced with these new teachers who act oddly. (The Doctor is replacing another teacher who suddenly won the lottery, despite the fact that she didn't play.) Then Finch comes in to introduce a Miss Sarah Jane Smith who is writing an article about the school. She shakes hands with the Doctor who claims to be a John Smith, Sarah says she knew a man who went by the name John Smith (referring to the Doctor). Although the Doctor immediately recognised her name, he does not let on to her that he is the Doctor.

Later, Sarah Jane is seen breaking into the school while, in another part of the building, the Doctor, Rose and Mickey are all investigating. While Rose collects a sample of the chemical from the barrel and Mickey investigates the maths department, where all the new teachers work, the Doctor decides to investigate Mr. Finch's office.

Sarah Jane, searching various rooms, suddenly comes across the TARDIS, parked in a small cupboard. Shocked, she leaves, only to find the find the Doctor standing behind her. She remarks that he has regenerated since she last saw him; he replies that he has done so "half a dozen times". Suddenly, a scream pierces the air and the Doctor and Sarah Jane run to find Mickey, who opened a cupboard, only to be covered in shrink wrapped rats. Sarah Jane comments that they must be for dissection; Rose retorts that rat dissection hasn't been used in schools for years, at the same time insulting Sarah Jane's age.