It follows three mummies as they end up in present-day London and embark on a journey in search of an old ring belonging to the Royal Family, stolen by the ambitious archaeologist Lord Carnaby.
The film follows the fun adventures of three ancient Egyptian mummies who live in an underground secret city, hidden in ancient Egypt. The trio includes a princess, a former charioteer, and his younger brother, along with their pet baby crocodile. Through a series of unfortunate events, the mummies end up in present-day London and embark on a wacky and hilarious journey in search of an old ring belonging to the Royal Family, which was stolen by the ambitious archaeologist Lord Carnaby.
Thut (Joe Thomas) is an Egyptian mummy and a chariot racer left with post-traumatic stress disorder after a crash in a chariot race. After the crash, Thut decides to retire from the sport. Sekhem (Santiago Winder) is Thut's younger brother who is skilled at boomerangs. Thut is against marriage as that entails living with one partner for eternity since mummies in the city of the Underworld live forever.
Meanwhile, explorer Lord Sylvester Carnaby (Hugh Bonneville), searches for the tomb of Princess Nefer. Carnaby finds the tomb, but her sarcophagus is found to be empty. In her chamber, Carnaby finds inscriptions for a city of the Underworld where the mummies go to live after the completion of mummification. Carnaby finds a hole in Nefer's sarcophagus, which he believes leads to the city of the Underworld.
One day Thut is crossing the street when Princess Nefer (Eleanor Tomlinson) almost runs him over with her chariot. After having an argument with Thut, Nefer reaches back to the palace and sneaks back in, assuming that she never took the Pharaoh's (Sean Bean) permission to leave in the first place.Nefer loves singing and the Pharaoh disapproves as he believes that singing is for entertainers and not for the rulers for the world.
Pharaoh wants Nefer to choose a suitor, but Nefer says that she is not ready to get married. Pharaoh releases a Phoenix bird to find a suitor for Nefer. The flaming bird soars above the city, when it is stuck by Sekhem's boomerang, and crashes on Thut's roof. The Pharaoh believes that Phoenix has chosen the suitor and summons Thut to the palace. Thut is told that he will marry Nefer in 7 days.Now Thut loved his bachelor life, and the marriage proposal comes as a complete shock to him. He has to agree as the punishment for refusal is cutting out of the tongue and gouging out of both eyes. Thut is given a Royal Ring to safeguard till the day of the wedding.
The Underworld city has an artificial sun, and the day and night cycles are manually controlled by operators. One night, Carnaby digs through the surface to find the City of the Underworld. Carnaby sends a drone into the city, who finds the ring box given to Thut and brings it back to the surface.Thut realizes that only the living could have stolen from them. The mummies had built the city to protect themselves from the living.Thut and Sekhem climb to the surface and Nefer follows them. They find Carnaby's camp and observe that he has the ring. They climb into one of the trucks being used to transport the antiquities. As the truck reaches port, Thut and Sekhem hide in one of the crates and are transferred to a ship. Nefer reveals herself to them in the cargo hold. The ship takes the trio to London.
Danny (Dan Starkey) and Dennys (Dan Starkey) and Carnaby's henchmen. Carnaby is terrified of his mother (Celia Imrie). Thut believes that they are in Rome, as that was the mightiest empire of their time.The trio follow Carnaby by hiding in the back of his pick-up truck. The mummies have to be careful as exposure to bright light reveals their Undead nature to the humans.The trio follow Carnaby to a theatre where he was watching a play with his mother. The trio are mistaken as cast members as the play is a musical about Egyptian culture. Nefer watches a ring in the hands of one of the actors on stage and starts singing herself to divert attention and to get to the ring. The ring was a fake, but Nefer's singing gains everyone's attention, including Carnaby.
After the performance, the trio try to commandeer a car to return to the Underworld. When Carnaby pursues them, they escape with Ed A. Murphy (Shakka), who is impressed with Nefer's voice. Nefer wants a career in music as back home, the only life for her is an eternity married to Thut. Sekhem reveals that he knocked the Phoenix down with a boomerang, so technically it never chose Thut. Thut and Nefer are relieved as their marriage is not foretold by the Gods.The trio stay with Ed for the night. The next day, Ed gives Nefer a makeover and then live streams Nefer singing. Nefer quickly gets 10 million followers on social media.
Carnaby realizes that Nefer is an Egyptian mummy who has come back to get her ring. Carnaby attacks Ed's apartment and kidnaps Nefer. Ed and Thut see the van driving away and Ed says that it belongs to the Carnaby museum. Carnaby chains Nefer inside his latest exhibit in his museum and plans to reveal as his greatest discovery to the world.Thut arrives at the museum, but Carnaby sees him coming on the CCTV cameras. Thut is captured as well.
Sekhem goes after Thut, when he doesn't return in time. Sekhem blacks out all CCTV cameras to enter the museum undetected. Sekhem manages to get the ring and save Thut and Nefer, but they are delirious from the effect of the sedatives. Thut and Nefer regain their senses as Ed arrives, and Carnaby again launches an attack, shooting Ed with a sedative.The trio escape with Ed and board a bus outside the museum. Sekhem takes control of the bus to escape Carnaby and his henchmen following in their van. Sekhem manages to trick Carnaby who drives his van on a ramp and onto a barge in the middle of the Thames.
Thut and Nefer start developing feelings for each other. Ed says that there is a flight to Egypt, that can get them home in 5 hours. Nefer decides to go back with Thut and Sekhem, to her people.On the plane's cargo hold, Thut admits that he is not brave and that he retired as he got scared. Turns out Carnaby is on the same flight back to Cairo.
The trio get back home, but both Nefer and Thut are sad. This time Carnaby brings excavators and enters the City of the Underworld himself. Carnaby goes after the Pharaoh himself. Nefer defends her father, and Sekhem encourages Thut to go help Nefer as their entire city is under attack.Thut gather his courage and attacks Carnaby on his chariot. The chariot is no match for the metal excavator. Thut manages to climb the excavator as it hurtles down a path towards a cliff. Thut pushes away the chariot with had Nefer on it and takes Carnaby with him over the cliff.But Thut manages to survive and emerges from the cliff with the ring. Thut proposes to Nefer, and she accepts. They get married in a lavish ceremony.
