Dr. Starfinder is working on a machine to "get around" the Fosters effect; little does he know that Foster comes to him. The students are informed of the Fosters Effect. This rule, discovered by Doctor Foster, prevents magic being used for selfish or trivial reasons. Mayhem ensues as the 'missing' Doctor Foster contacts the college after having gotten caught in a spot of bother.
Ben is trying to fix the coffee machine, and Milly is helping him. She tries a spell to help, but a load of coffee grains fall out of the machine and splatter onto Ben's face. Starfinder is talking to his class about the Foster's Effect. Tim and Azmat aren't listening, so he calls Tim up to the front of the class, and drops a slice of buttered toast onto his hand. Milly asks Ben why he's going away, and he says to study rocks. Milly looks over at two pretty girls sat in the café – he is going away with them, and there will be other girls from his college there too, and Milly is jealous. Starfinder is lecturing about Dr Foster, and the Foster's Effect. They discuss the likelihood of there actually being a real Dr Foster – Azmat thinks there is, but Starfinder thinks there isn't because no-one has heard from him for 30 years. Tim and Azmat show Starfinder a magazine article, which he reads.
The teachers are discussing the staff supper when Starfinder walks in. He shows them the article about Dr Foster being kidnapped because he has been rumoured to find a way of getting around the Foster's Effect (the law that magic rebounds on itself). Thunderblast is convinced that it is a spoof, but when the place shakes and they hear a voice saying "Mayday mayday Foster here, I'm being kidnapped," she is forced to change her mind. Starfinder suggests building a machine to summon Foster. They need something of the Doctor's – Professor Shakeshaft tells them that he can help.
Cas is concerned about Dr Foster, but all Milly cares about Ben. She thinks about casting a spell on him to make him think about her all the time while he is away. Starfinder is telling his students what to do as Shakey appears with a letter sent to him by Foster thirty years ago, when the Effect was first discovered. They locate Foster using a crystal – it turns out that the Doctor is in Australia. They cast the spell. A woman appears, and they come to the conclusion that she must be his assistant or cook. Jenny Wendle asks if Foster has been kidnapped, and she says that Foster has. The woman, who's name is Josie, asks for some tea, and Thunderblast takes her for some scones.
Cas walks in on Milly making a Forget-me-Not Potion for Ben. Cas says it's unfair on Ben and she shouldn't use it. Josie and Thunderblast are in the office, talking about Starfinder and Fosters' kidnap. Starfinder appears and asks Josie if Foster had a machine and whether she knew what Foster was working on. He suggests a magic Hypno Potion to help her remember – she agrees to help them.
Hobbes and Milly are in Misery's talking about how they are going to get in on the project. Milly pours her potion into Ben's cup, but Ethel and Hobbes see her. Ethel has an idea. Ben is about to leave, and they are about to drink their coffee when Ethel comes up to the couner. She manages to distract them and she switches their cups around. Milly drinks the cup with the potion in.
Starfinder and Josie are working out the machine – she can't remember anything about it and Starfinder tries to help her along. Starfinder leaves the room to get people to help. In the Ambi-Lacrum, they are all working on the machine. Milly can't keep her mind on the job, as she is thinking about Ben. She accidentally pours the potion over a circuit, and Thunderblast suggests that she goes and looks after Josie. Milly wanders into the room with tea for Josie, and she can't stop thinking about Ben…Josie suspects something is up, and tells her that she's been potioned. Milly realises that Ethel switched the cups. Josie tells her that it will wear off eventually, and starts explaining the Fosters' Effect to her to distract her. Tim and Azmat burst into the room to tell Josie about their plan for an Anti-Magic Potion, which sounds more like a soup!
Shakey comes in to tell Josie about Hobbes and Ethel being interested in the machine. Milly falls asleep. The machine is finished, and Josie comes to inspect. Hobbes and Ethel tell Josie that there's something wrong with a lever, and Josie realises that they can't have Foster's Free Magic without a Foster's Free Magician. Hobbes and Ethel drink potions to turn them into frogs while Starfinder starts the machine. The ball rolls down the track, and leaves a trail of flowers behind! Starfinder realises that Josie has been leading them on from the start, and Shakey reveals that Josie is in fact, Dr Foster. Milly remembers that she was supposed to be at Misery's catering for the Staff Supper, so there is no food for them, but the soup that Tim and Azmat have brewed up is perfect!
Josie explains that she gave up magic after the Foster's Effect, and that it was the backwards Fosters' Effect that made them hear her voice before they'd kidnapped her! Ben comes home early from his trip, and tells Milly that he came home because he couldn't stop thinking about her.
The 7th episode in the series (Dr. Foster, I Presume) is a reference to the quote "Dr. Livingstone, I presume" by Henry Morton Stanley.