A darkened room, windows to high for a human to see the three red moons in the night sky, comes to life briefly. For a split second there is a sensation that it is full of light and hot sweaty bodies. Mostly, however, it is silent and the impression that there is a man with a bow tie and a tweed jacket bound hand and foot in the room seems like a fleeting moment of horror.

Andrew Brown works for Lexington International Bank. He has worked hard, built a career and risen up the ladder. The only impediments to his continued career rise are his rival, Sameera Jenkins, and the fact that he never has enough time to complete the work that would ensure his next promotion. Aggravatingly, Sameera always seems to manage her time better than him. On this particular day, Andrew is hitting one stumbling block after another: his phone is dead, he has overslept, he has spilled water over his suit, forgotten his sister's birthday and his train is late. As a result, his presentation speech goes badly wrong. When he returns to his desk he is approached by two strangers, Mr Symington and Mr Blenkinsop. They tell him that they represent a consortium that can lend him time. He tries to get rid of him but they provide him with a demonstration that shows exactly what they mean.

Amy and Rory spend a holiday on fifty-first century Earth. They are currently in a Super Lucky Romance bubble - a device that slows time down so that they can enjoy precious moments for longer. Unfortunately they haven't read the signs on the beach and find their tent infested with flying crabs. As the crabs double and redouble in number, Amy phones the Doctor. The TARDIS materialises beside them and the Doctor rescues them from the bubble. He presents Amy with a tulip as a wedding gift. When she fails to be impressed he explains that it represents the folly of humans who thought that they could sell it for more than it was worth. When that still doesn't stir her interest he decides to take her to the Lexington Bank to witness the greatest banking collapse in human history.

In London's Square Mile is the Lexington bank. Down in the basement of the bank there is a maze of sore rooms and this is where the TARDIS materialises. Amy and Rory emerge wearing business suits (or as near as they are ever going to get). The Doctor leads them to a street level door and they walk out onto the pavement where they see a dirty old woman huddled on the floor. She is wearing expensive clothes but seems incredibly tired. Rory gives her a sheath of banknotes and she tells them not to go back into the bank because they will have their time stolen. When the Doctor asks her what she means the woman snaps into business mode and presses her card into his hand. It tells him that she is Nadia Montgomery, Head of Communications and Marketing. The Doctor thanks her. Rory7 and Amy think that she is mad but the Doctor treats her with respect.

The trio enter the bank where the Doctor announces himself to the receptionist as Doctor Schmidt, the efficiency auditor from Zurich. Amy is passed off as his assistant and Rory is sent off to work in the mailroom. Vanessa Laing-Randall takes them on a tour of the building. On the way she shows them a huge glass sculpture that twists its way up through the central well of the building, reaching as high as the eighth floor. She says that it is their mission statement: each strand represents the work-life balance.

Amy looks around at the different offices visible from the balustrade where she is standing. The building reminds her of a bee hive. Suddenly he fiddles with something on his wrist and collapses to the floor of his office. She is sure that he is dead.

By the time they get there, the man's secretary has covered his body with his jacket. Laing-Randall says that he died of overwork, when the Doctor examines the man there is no evidence of a watch on his wrist. Despite this, the Doctor tells Laing-Randall that she is messing with dangerous forces. She is perplexed by this idea and invites him along to her next meeting.

In the mailroom, Rory is puzzled at how much work is being generated by Andrew Brown and Sameera Jenkins. He asks a few questions and finds that some of the staff are said to be able to be in New York and Tokyo on the same day. Two of the other workers hint, with amusement, that he can find answers in Storeroom F. When he gets there he feels an odd, tingling sensation. The door slams behind him and there is a fluttering noise. He has to use his mobile phone as a torch. In the dim light somebody knocks boxes flying, tugs at his jacket and chuckles. A see-through woman flickers into being and disappears again, several times.

Spooked, Rory returns to his work station in the mailroom where he is astonished to find himself taking a phone call from Andrew Brown on the fifth floor about a package while he can clearly see Brown in the mailroom berating another worker.

In the boardroom, waiting for a conference to start, the Doctor is taking readings. He asks if anyone is using a temporal inhibitor. The executives stare at him blankly. He settles down to watch Andrew Brown's presentation, delivered well enough. However, the clients' request for details on the tax implications means he has to leave the office for a moment, returning almost instantly with a document that covers the problem. A similar thing happens when Sameera is asked to find an in-house lawyer and seems to have one waiting ready outside. This pattern is repeated throughout the meeting until the Doctor flusters Andrew by asking when he had the time to suddenly change his tie. The Doctor drags Vanessa Laing-Randall into the corridor and asks her what is going on but she still seems unaware that anything odd is happening. He accuses her of using time travel technology but this puzzles her even more.

Amy is finding out about 'stuff'. She has been told by the Doctor to stay close to Sameera Jenkins, an Asian woman with a Lancashire accent. Sameera criticises the length of Amy's skirt before going off to a meeting. Amy searches Jenkin's office for a bracelet but finds, instead, receipts that show she ate five lunches the previous day. Sameera returns to the office and Amy sees that she is, indeed, wearing the bracelet. Amy lunges for it, Sameera presses a dial and then everything changes. Ghosts of the pair of them can be seen flickering round the office while Sameera wails that it is out of sync. She readjusts the watch and they flip back fifteen minutes. Sameera says that she only uses time travel for work because there is so much to do. Amy says that she is a time traveller, too and Sameera asks who she borrows her time from.

Sameera arranges for Blenkinsop and Symington to explain it all to Amy. They arrive remarkably quickly and explain the concept of borrowing time. They give her a bracelet. Two phone calls arrive from the Doctor and Rory and Amy decides to try it out. They tell her that the interest repayment is five minutes per hour. Amy has a leisurely lunch with Rory and then goes back in time to meet the Doctor. Just for fun she winds back an extra two hours and goes out for a haircut and a manicure. She does a quick calculation and sees that she owes the watch twenty five minutes interest. She decides to borrow a whole day and hires a car. She drives back to Leadworth. She borrows a bit more time to enjoy her day with her parents and then drives back to London to get in several hours shopping before it is even ten o'clock. Realising it is twenty five hours since she had any sleep she winds back again and books into a hotel.

When she returns to the Doctor she realises that she has borrowed about four days in total. He senses something as soon as he meets her and pushes up her sleeve to reveal the device. She feels ashamed. He tries to get it off but it is locked onto her wrist, woven into her persona. Amy tries to pacify the agitated Doctor by saying that she has only borrowed four days, accruing eight hours of interest. The Doctor says that she isn't paying the interest on what she borrows but also on how long she borrows it for. With the aid of a cake to demonstrate, he points out that she already owes ten years of her life. Amy tries to prise the bracelet off and Blenkinsop and Symington appear. They point out that removing the Time Harvester will mean forfeiting the rest of her life. The two men bare their teeth like sharks. Amy suddenly realises that the curious humps on both men's backs are actually shark fins. Amy and the Doctor run, pursued by the two humanoid sharks.

They jump into a lift but when they reach the ground floor the two shark-men are waiting. They evade them and run on past where Amy and Rory are still having lunch. Rory immediately grasps that this is a 'time-travel thing' and runs with the new Amy. He uses the Super Lucky Romance Camera to create a bubble around Blenkinsop and Symington. They stroll back to the TARDIS with the Doctor but find that three pairs of Blenkinsop and Symington are waiting for them on the pavement outside.

Amy reaches into her bag and is surprised to find a spanner. She throws it at one of the shark-men, causing a gash in his head. Rory notices hat all of the other shark-men has a gash on its head. The old woman, Nadia, that they met earlier, steps into their midst. Rory sees that she has a watch like Amy's except this one is fizzing like a firework. As the shark-men rush towards her Rory uses his camera to catch them in time bubbles. Nadia holds out her wrist and tells the Doctor that the watch is hurting her. The Doctor says that it must be taking and returning time randomly. He says that he will try to make it stop.

In return she goes to the room where the TARDIS is and finds it surrounded by dozens of Blenkinsop and Symington ghosts. The Doctor leads Amy and Rory through London. He says the fact that all of the shark-men were gashed on the head means that they are all the same creature at different points in time. If one of them sees them then all of the others will, too. He fires a sonic burst at Amy's watch, saying that this will send some of her time back to the bank and convince Blenkinsop and Symington that this is where Amy is now. This should buy them some time.

The Doctor says that they need a friend in the bank. Rory suggests Andrew Brown because he must have borrowed so much time. The Doctor rings Galactic Enquiries to get Brown's address and they make their way to his flat. They find him simultaneously cleaning, painting, trimming, cooking and playing. Somehow all of these versions remain oblivious to the others. The Doctor approaches the one cleaning the car and asks how much time he has borrowed. Andrew says that he has tried to pay back the time as soon as possible and only owes 'a couple of weeks'. The Doctor uses his screwdriver to show that Andrew owes 55,000 years. The Doctor wonders how the lenders can ever hope to recoup the time that they are owed.

The Doctor says that time is too volatile to transport so it must be stored on Earth. He thinks that it will be in the Bank but Andrew has other ideas. He says that he has noticed unusual invoices from Little Green Storage in the Millennium Dome.

They go to the Dome and find a small door with Little Green Storage scrawled on it. The Doctor rings a bell. Somebody who recognises him from a different incarnation invites him in to visit his storage locker. When the group step through they find that they are in an enormous space: five miles across and several hundred feet high. The walls are lined with doors. A small figure floats beside them. The Doctor introduces him as the Yomalet-Ram and says that they have all been shrunk to a fraction of their size to fit into the Dome. When the alien leaves them they travel in an elevator to the Lexington Bank locker. Unfortunately, it stops near about forty of them. The Doctor produces a master key and they start searching for the right one. Most are full of alien technology, including a TARDIS shaped console being repaired by red metal robot men. Eventually they find the right one. It is lined, floor to ceiling, with green glass boxes on shelves.

Nadia watches the shark-men attach a glass sphere to the TARDIS door. It is some sort of Time Bomb. She knows she must stop them gaining entry.

Amy asks if the glass bricks, each labelled with a name, contain the borrowed time. The Doctor says he would be able to feel it if that were the case. He suspects that the bricks are more like bank statements. Andrew Brown finds the one with his name on it and takes it. The cockroaches, which have been scuttling about, start to grow to the size of large dogs. The Doctor leads the escape back to the elevator but, when it won't take them back to the exit, he decides to take them to his storage locker.

The Doctor's door has a warning: 'Don't come in here - seriously'. They enter anyway before the car-sized cockroaches catch up with them and find themselves crushed into a tiny vestibule. The Doctor tries to open the interior door but a message pops up on a dot-matrix screen telling him not to enter. Instead, it opens a hatch and gives him a magazine article, some batteries and some bug spray.

Nadia enters a store room and picks up a telephone directory. She winds time back by an hour and then waits for the Blenkinsop to arrive with the Time Bomb. She hits him over the head with the directory and then retrieves the bomb as he drops it. She makes her way outside, now ageing faster than she ever has done.

Andrew winds back time to create more and more versions of himself, each armed with bug spray. He opens the door and launches an assault on the giant creatures. When they are all dead, Yomalet-Ram appears. He says that, despite the Doctor being such a lucrative client, he couldn't be shown any favours. He guides them to the exit. Outside, returned to normal size, they find it is daylight. Andrew decides to smash his glass brick to find out what will happen, but Rory stops him. He shows him a note on the underside saying that the brick should be returned to the office of Vanessa Laing-Randall, Lexington Bank.

The only one who can re-enter the bank safely is Andrew. He wants to warn the others about what is going on. The Doctor wants to find Nadia and look at her watch. Andrew leads Nadia to the Doctor in Temple Gardens where the Doctor tries to get her watch to emit a field that will cover all of them so that the Blenkinsops and Symingtons ignore them like they do Nadia. As he does so, Andrew returns to the bank and begins to explain to Sameera about the watches and the accounting system. Sameera has been very careful with her watch but still owes 35 years.

The Doctor shows Rory the magazine article, dated 5013. It is about the discovery of Cosmic radiation Power at Aberdeen University. The Doctor explains that the article is sealed inside a time-bag that keeps it impervious to fluctuations in time. When he removes it from the bag the details have changed and the discovery was made in Tokyo. He says that this proves that the planet is losing its future. He hands the batteries to Rory to use in the Super Lucky Camera to replace the infinite power it originally had before the change in the time stream.

Sameera and Andrew work their way through the offices, talking to all the people that have been using the watches. They are too late to save one man who tries to pay back his debt all in one go and dies instantly. Sameera and Andrew realise he is the tenth person to die in the bank in the past six months. They decide to stop competing with each other and to work together. They conclude that the place where the Blenkinsop-Symington's seem to emerge from is Vanessa's office.

The Doctor says that the only way they can move about is if they take Nadia with them wherever they go. They steal a wheelchair from St Bart's Hospital to transport her.

When Andrew and Sameera get to Vanessa's office they find her mousy secretary, Jane Blythe, in the outer office. She hovers nervously while they search Vanessa's files but find no evidence. They tell Jane that the Doctor has hinted at alien involvement. A Blenkinsop-Symington arrives and says that such behaviour means the forfeit of all owed time. Jane slams the door shut, with her, Andrew and Sameera on the inside, as the pair begins to transform into sharks. She tells the two humans that all of the bad things started when Vanessa was transferred to London. Sameera says that they need to talk to the Doctor and Amy because they are time travellers. The shark-men batter the door down and Andrew and Sameera escape past them leaving Jane inside.

The Chancellor is preparing to make a speech from a podium erected in front of television cameras in the bank. Sameera and Andrew find the Doctor back stage. He is with Amy and Rory as well as Nadia (who has reverted to being a child since the Doctor's latest attempt to tamper with her watch). As the broadcast goes live, Amy rushes the stage to say that aliens have been giving out watches like hers. She brandishes it at the camera before pulling out the ten-year old Nadia to tell her story. Nadia's watch stops; the protective field collapses and dozens of shark-men turn their heads to look. Sameera weighs in by stepping into the camera's vision and paying back her thirty five years in one go.

Among the confusion and panic, the shark-men eventually trap Amy and Rory. They threaten to make Amy pay back her time debt but the Doctor steps in and says that he will stand in for her. Vanessa arrives on the scene. She has no idea what is happening and says so. The Doctor agrees but looks at Jane, adding that she is the mastermind behind the organisation. One of the Symington's bites Vanessa on the shoulder, draining the time out of her until she empties like a husk.

Jane confesses that she is like a tree trunk and the Blenkinsops and Symingtons are her many branches. The Doctor looks away, apparently bored and comments on the sculpture that dominates the atrium. He casually mentions that Jane hasn't called in much of the borrowed time. He then reveals to Jane that he is a Time Lord and offers to pay Amy's debt, but only after Rory has made the same offer. The watch falls from Amy's wrist and shatters. The Doctor says he will take on other debts and removes Andrew's watch. He puts it on while whispering to Amy that the sculpture is the Time Harvester liquidity fund.

After the shark-men lead the Doctor away Amy tells the others left with her that they need to smash the sculpture.

The Doctor is taken to a large, dark hall, which is intermittently (and only for infinitesimally small periods) a hive of activity. For an appointed time the room is filled with glass bricks and the screens flicker into life. Traders around the galaxy focus on Jane Blythe as she offers to auction the last Time Lord.

Amy, Rory, Nadia, Andrew and Sameera drag a filing cabinet to the edge of the atrium above the sculpture. They are blocked by the shark-men but Amy grabs the Super Lucky camera and presses the shutter while running, thus enclosing herself and her friends in a long time bubble. It gives them three hours of subjective time to make their preparations. When the bubble collapses the shark-men close in but the filing cabinet topples onto the sculpture which disintegrates. The Blenkinops and Symingtons disappear.

The Doctor addresses the traders in the Time Market. He points out that Andrew Brown owes 100,000 years of time. Then he asks the traders to check their data bases to see how long a normal human actually lives for. There is a period of panic, the market becomes unstable and Jane vanishes. The Doctor steps in to make an offer and buys the remaining time bricks for one second per decade.

When the police arrive to find out how the vast sculpture was destroyed it seems that no-one can remember. The owners of the Time Harvester watches find the displays say that the contracts are now void. The Doctor reappears and hands Sameera and Nadia their time bricks. He says that the owners can open them. When they do, Sameera returns to being 35 again, though Nadia stays 20 (despite actually being twice as old).

After the TARDIS departs, Sameera says that she wants to open a delicatessen somewhere and Andrew thinks he would like to be a teacher. They embrace.

The TARDIS arrives in Foreman's Yard, Totter's Lane, in London's Isle of Dogs. The Doctor tells Rory that Lexington Bank will still collapse but Nadia will be able to rescue some of it. He walks to a warehouse and knocks on the door. A voice from the dim interior greets him and says that one day he will become a valuable client. Or already is. The voice asks if the Doctor needs something secreting amongst another client's belongings.