Chain Reaction ( audio story ) Chain Reaction was a 2011 Big Finish Productions audio short story . It was read by Louise Jameson and featured the Fourth Doctor . Summary . One single coin can set off a fascinating sequence of events . . . if you ' re a Time Lord . Plot . The Fourth Doctor is sat outside a shopping centre . He is preparing to toss a coin when he interrupted by a security guard , Dennis Brown . While they talk , with Brown instantly taking the Doctor for a lunatic , a cyclist some distance away is covered in paint as a workman knocks a pot of premium gloss over her . Now , back at the original starting point , the Doctor rolls the coin towards a pigeon , undisturbed by Dennis . It flies off and a teenager comes along and , seeing the coin , stoops to pick it up . A businessman , engrossed in his newspaper , promptly trips over him . The coin lands in the fold of the newspaper and when the businessman picks it up , flies out and jams in the spokes of a woman ' s bicycle . This time , the paint spills harmlessly . The Doctor begins to untie one of three knots in his scarf . Meanwhile , the cyclists dismounts and moves to pick up the pound which has settled in a delivery bay . As she does so , a " Sam ' s Sandwiches " van sounds its horn , startling her . She moves , signals her apologies and cycles on . The coin is under near - side back tyre . Sam steps out and fetches sandwich s from the back . Across the road , Emma tries to avoid staring at him . He greets her as he passes into an office building , prompting her to blush and drop the pound coin she was putting into a parking meter . The coin rolls into the road where a passing group of children inadvertently kick it further along . Emma does not see this and believes it is under the van and so stoops to look . Sam comes back and greet her again causing further blushes . The two laugh and the circumstances and the Doctor unties the second knot in his scarf . Emma ' s coin rolls into a builder ' s trench , lands on a workman ' s spade and is batted into a long plastic pipe being hauled on a winch up the wall of the shopping centre by a second builder . The pipe is not straight , meaning that the coin rolls towards the end closest to the Doctor , gaining momentum . The Doctor stands to watch proceedings and prepares to untie the third knot , but expresses displeasure when he sees Dennis approach to collect litter and stop to talk to the builder . The conservation does not end amicably , as the Doctor can tell , with the workman turning away to resume his task . The pound falls from the pipe into a cement mixer . The Doctor releases the knot and , looking to the sky , asks if he is being tested . Again , the Doctor returns to the original moment . He triggers events once more with his coin and counts off each as they happen . The pigeon flies off , the teenager collides with the businessman , the cyclist allows Sam to park and Emma and Sam arrange a date in the local Italian restaurant . As Sam drives off , Emma finds the Doctor ' s coin . The Doctor again unties the second knot . Emma ' s coin goes from the spade to the pipe . This time , Dennis does not appear so the worker is not interrupted . The pipe is winched higher , so the coin descends in a greater arc . It lands in a market stall awning and bounces into the sweet stall opposite , landing in the cash register . Startled , the stall owner knocks a table and a packet of sweets fall into a passing woman ' s handbag . The Doctor deftly lifts the sweets from the bag as the woman ' s handbag , remarking that they were " bought and paid for " . He unties the third knot . Dennis approaches the Doctor and enquires is he well . The Doctor tells him that the coins were fixed in space - time and tells Dennis the fact he is not is highly unusual . Sarah Jane approaches and , noting the security guard , asks the Doctor if he is in trouble . The Doctor replies that he was telling Dennis how amazing he is , being " almost immune to time " . As they walk away together , Sarah asks if he was bored . The Doctor tells her he kept himself amused and offers her a jelly baby .