At the entrance of the Casino Majestique in Monte Carlo, Jean, the Commisionare is welcoming Lady Lily Hawthorne and other guests to the casino. Peri and Erimem are outside discussing their fancy dress outfits and what it is the Doctor wants them to do, which is to prevent a robbery. He, meanwhile, is arguing with a small man in a strange pullover who keeps looking at both of them in a funny way. The Casino is hosting a party to see the Veiled Leopard, a diamond Erimem buried in Ancient Egypt, something she reveals to Peri just before they enter the casino.

Inside, Lily is socialising with various other guests, including Gavin Walker, an industrialist and host of the evening's event. A friend of Walker's, Peter, is in a similar line of business. Both have worked for their status in society, and were not born into like many of Walker's guests. Peri and Erimem enjoy a glass of champagne while Erimem begins to explain her connection to the diamond, but is interrupted by Peter, who doesn't impress Peri. Lily comes over and saves them from Peter, immediately making a good impression on Peri.

Lily introduces them to Walker whom Erimem immediately dislikes by not only dressing like her father, but then insulting his memory by not knowing his name. Before Erimem gets too enraged and gives away too much about her origins, Peri makes their excuses to Walker and Lily introduces them to other guests.

Lily makes her excuses and leaves Peri and Erimem on their own, giving Erimem the chance to explain that the Veiled Leopard was buried with her father; that Walker now owns it means that her father's grave was desecrated by tomb robbers. Lily returns and tells them that the diamond is an office safe upstairs and that she hopes the society thief Janus, known by that name because the thief's calling card features an image of the Roman God Janus on it, will steal it.

Walker makes a speech welcoming the guests and revealing that the Veiled Leopard will be given to his wife as a birthday present at midnight. Erimem points out the security guards Walker has hired to protect the diamond, and conversation turns to Janus and if the thief is at the party, which guest they could be, with their attention turning to a stranger. When Lily goes off to answer a phone call, Peri and Erimem follow the stranger to see him enter the Gents. Just then, Erimem notices that Peter appears to be standing guard outside the room Lily entered to take her call; Peri's suspicions are also raised when he blatantly asks another guest, Hex, to light his cigarette. Both quickly come to the conclusion that he and Lily are working together, and then learn from the Comissionaire that the directly above the room Lily is in is Walker's private suite.

Peri and Erimem make their way up a flight of stairs, and discover that the guards outside Walker's private suite have been drugged. Erimem uses a set of skeleton keys to get inside the suite. Walking around and getting use to seeing in the dark, they soon find the room in which is Lily in the middle of stealing the diamond. Lily reveals that she only steals from those whom have made or increased their fortune through underhand tactics; the stolen items are sold and the money is distributed to those in need. Erimem debates over allowing Lily to steal the diamond as it would be for a good cause, but Peri reminds her of their mission. All discussion ceases as they hear someone, Hex, outside the room. Lily makes her escape the same way she came up to the suite, the dumb waiter, while Peri and Erimem use the laundry chute to make their exit, just as the door opens.

At the entrance of the Casino Majestique, Jean, the Commisionare is welcoming Lady Lily Hawthorne and other guests to the casino. Ace and Hex are preparing to enter the party, with Ace in charge of their mission to ensure that the Veiled Leopard is stolen by the right person. The Doctor has informed them of the relevant people to watch at the party, and both are wired up with ear pieces and microphones.

Inside the casino, Lily and Peter discuss their plans, before returning to their respective cover stories. Ace and Hex enter the party, as Ace reveals to Hex the importance of the diamond. A civilisation on a dying planet encoded the diamond with all the information required to recreate their entire planet on a new world. While being transported to the world where the civilisation would be rebuilt, the ship was destroyed by other races after the technology to encode onto crystal. The Doctor rescued the diamond and put it on Earth until the time was right to take it to the new planet; due to how the data is encoded the diamond cannot travel in time. Now it is the time to restore the civilisation.

Ace and Hex circulate in different parts of the casino, with Hex keeping an eye on Lily, currently talking with Peri and Erimem. Walker makes a speech welcoming the guests and revealing that the Veiled Leopard will be given to his wife as a birthday present at midnight. While Hex keeps an eye on Peter, Ace follows Walker. Hex finds Peter standing guard outside the room where Lily is taking her 'call', Peter blatantly asks Hex to light his cigarette. Ace overhears Walker's conversation with Bennett, a thief, whom Walker is getting to steal the diamond before midnight, with the crime to be blamed on Janus. Walker threatens Bennett about what would happen if he doesn't do as instructed.

Hex heads upstairs to Walker's private suite, while Ace flirts with Walker in an unsuccessful attempt to get a private viewing of the diamond. Hex discovers the unconscious guards and breaks into the private suite, disturbing Lily, Peri and Erimem in the other room. As Lily, Peri and Erimem make their escape, Hex hides as Peter enters the suite, looking for Lily. Hex confronts Peter and explains that he's here to help. Although suspicious, Peter goes along with it when Hex helps him break into Walker's safe. Hex gives Peter one of two copies of the diamond, with the other going in the safe and Hex taking the real Veiled Leopard. Hearing noises outside, Peter and Hex hide while Bennett breaks in and takes the fake diamond.

Ace enters the private suite and tells Hex and Peter that Bennett is escaping. Peter takes the fake Janus calling card placed by Bennett out of the safe while Ace and Hex use the back stairs. They return to the front entrance of the casino as Peri and Erimem make their way back inside to enjoy the rest of the party. Hex and Ace decide to go after Bennett, while Peter explains his actions to Lily.