Build a Better Mousetrap
by Brian Clemens
Designed by Douglas James
Directed by Peter Hammond
Mrs Gale joins a motorcycle gang, proving herself by doing a 'ton-and-ten' but when she goes to a water mill to ask for directions, she and Dave (Donald Webster) are sent away with a flea in their ear by the two elderly sisters who live there, who threaten to put a spell on them.
Ermyntrude (Nora Nicholson) is quite anxious about the situation, but the vivacious Cynthia (Athene Seyler) heads back behind a curtain where her life's work is hidden... and all the motorcycles stop working.
Steed installs himself at a nearby pub, run by the morose Harris (Harold Goodwin). While waiting for Stigant (Allan McClelland), a scientist from a nearby nuclear plant, he meets Colonel Wesker (John Tate) and his daughter Caroline (Alison Seebohm) who are also staying there. Stigant arrives and tells him the plant has nothing to do with the mechanical failures, but he'll investigate.
The cycle gang arrive and head into the bars. Steed befriends their leader and cons the cycle gang into riding a point-to-point around the area, and uses the resulting mechanical failures to pinpoint the location of the engine-jamming - the old water mill!
Steed visits the mill, posing as an inspector from The National Distrust ("... You see the National Trust trusts people to look after buildings of historic interest - but we don't - we don't trust anybody - far from it - that's why we inspect, National DIStrust, you see?"), but he is shooed away having learnt nothing other than they are suspicious - as he leaves, Dave appears and investigates the mill. Steed later arranges to meet Stigant at the mill, who has learnt that the women are Professor Peck's daughters, but when Steed arrives he's set upon by Gordon (David Anderson), who has already killed Stigant.
Steed retuens to the pub and informs the authorities of Stigant's death. Colonel Wesker blames the bikers for the murder while Harris declares he was killed with witchcraft. Cathy arranges with Dave to investigate the mill, while Steed waits for a telegram from London. Dave's girlfriend, Jessy (Marian Diamond), is sulky about him going away with her, suspecting other motives. Cathy tells her she has the wrong end of the stick and , unable to find Dave, goes to her bike. As she gets on, he leaps onto his bike and they ride off, bringing Jessy to tears. Harris then clears the teenagers out of the barn and makes a pass at Jessy as he tidies up. Steed plans to follow, but a simpering Caroline intercepts him at the bar and drugs his whisky. Steed has been suspicious of her for a while and pours the drink away, then pretends to pass out. She says "Niet!" as he raises a hand for help and leaves.
Steed makes Jessy read his telegram to see what's really going on - the Professor had developed a mechanical jammer before he died, and his daughters are confirmed to be living in the mill. Meanwhile, Cathy finds Cynthia's device and her companion reveals himself to be Colonel Wesker, who pulls a gun on her.
Harris then finds Dave bound and gagged in the loft. Steed realises that Cathy's in danger, and rushes to the mill with Dave.
Back at the Mill, Gordon and Caroline arrive and they prepare to leave, but the sisters return home and Cathy attacks Gordon. Steed and Dave turn up and an almighty fight breaks out. The Colonel escapes, not with his desired jamming device but with Cynthia's life's work - a better mousetrap!
It explodes outside, and he's captured, Steed commenting that it seems to be equally effective with the larger rodents.