Mr. Whittaker traveled back in time to meet one of his ancestors, a man named Albert. After Mr. Whittaker returned to the present, he began finding mysterious letters in the Imagination Station. The letters ask for items to save Albert from being locked in a tower. Unfortunately the Imagination Station has stopped working for Mr. Whittaker, so he asks two cousins, Patrick and Beth, to help him find the items. They have retrieved a Viking sunstone, a silver chalice and the tablet of Kublai Khan. When they attempt to return to Whit's End after their latest adventure, however, they find themselves in a dark cave. They are uncertain of where they are or the time period.
Patrick and Beth begin to make their way out of the cave but are confronted by a boy holding a knife. He is initially worried that Patrick and Beth will attack him. Then he sees the golden tablet of Kublai Khan and thinks they are thieves. Patrick and Beth explain that the tablet was a gift from a Mongol princess. The boy asks if they are friends of John Whittaker and introduces himself as James. James explains that he wrote the letters requesting help for Albert. He would leave them on a rock in the cave and they would simply disappear.
Patrick and Beth offer the tablet to James. He thanks them and then explains Cleopatra's crown is also missing from Lord Darkthorn's collection, so Albert is still in danger. Apparently the treasures disappeared around the same time Mr. Whittaker left. James wrote the letters believing that Mr. Whittaker had stolen the items and would return them if he knew Albert was in trouble. Patrick and Beth tell James that Mr. Whittaker didn't steal the items, but found copies of them, with the cousins’ help, because he wanted to save Albert.
Suddenly the children see a flash of light. The knight who saved Patrick and Beth from the giant eagles in China appears. He is badly wounded from the fight with the birds and immediately collapses. The knight instructs them to help him into a secret room within the cave. After they get him into the room, Sir Andrew, the knight, explains that he is able to jump through time in the Imagination Station by putting on a ring that Mr. Whittaker gave to Andrew's brother Albert. Patrick and Beth look around the room and see the Viking sunstone and silver chalice they retrieved to help save Albert.
James and Patrick begin to undress Sir Andrew to clean his wounds, so Beth goes back into the front part of the cave to give them privacy. Soon she hears two men approaching. She tries to warn Patrick, James and Sir Andrew, but the strangers enter the cave before she has time.
Beth hides behind a large rock and waits to see what will happen. The two men find the secret room and return, holding on to Patrick and James. James protests, but one of the men, called Hugh, states that it is his duty as Lord Darkthorn's steward to catch thieves. The men had seen the treasures hidden in the secret room and assumed they were stolen.
After the men leave with Patrick and James, Beth goes back to the secret room. Sir Andrew is gone, and Beth thinks he must have used the ring to jump through time. James was forced to leave his knife behind, so Beth picks it up and puts it in her pocket.
Suddenly Hugh returns. Beth quickly hides behind a shield and watches Hugh pack up the treasures. When he leaves, she follows him. Hugh goes to a cottage in the woods where he dumps the treasures on a table. Beth sees him comparing two different sunstones, chalices and tablets. She also sees Cleopatra's crown and realizes that Hugh is the thief.
Beth waits for Hugh to leave and gathers the extra treasures. She wants to find Albert to tell him what she'd seen. On her way to Lord Darkthorn's castle, Beth sees James and Patrick, who were locked in the stocks for being thieves. Beth rushes to help them. James picks the lock with the knife Beth gives him and then takes Patrick and Beth to see Albert.
Beth explains everything to Albert and gives him the three treasures, but Hugh and his guard, Roderick, suddenly barge into Albert's room. The children quickly hide, but Beth reveals herself in an attempt to keep Hugh from noticing the treasures she brought for Albert. Hugh finds the treasures anyway and threatens to lock both Albert and Beth in the castle tower. Patrick and James try to stop Hugh from taking Albert and Beth, but Hugh calls for more guards, and the two boys are overpowered.
While Hugh is escorting them across the castle grounds, Sir Andrew appears. Hugh explains that he is locking Albert, Beth, Patrick and James in the tower for being thieves, but Sir Andrew says only Lord Darkthorn can lock people in the tower. Hugh is outraged and challenges Sir Andrew to a joust.
Sir Andrew accepts, though he is still severely injured. While Sir Andrew is preparing for the joust, Beth asks about the white rose on his shield. He explains that it symbolizes his loyalty to the Duke of York, who should be the rightful king of England. He says the enemies of the Duke of York wear red and that their symbol is a lion.
When Sir Andrew's challenger appears, he is wearing red armor, which indicates that he has betrayed Lord Darkthorn and the Duke of York. Sir Andrew realizes that Hugh must have been stealing the treasures to raise money for the enemies of the Duke of York. The joust begins, and Sir Andrew takes a heavy hit from his opponent's lance. They charge each other again, and Sir Andrew manages to knock his opponent off of the horse. The joust should be over, but the red knight refuses to accept defeat. The injured Sir Andrew falls off of his horse, and the red knight attacks him with a sword.
Sir Andrew's horse kicks the red knight. He decides to mount his horse and charge Sir Andrew with another lance. Patrick and Beth grab a shield and place themselves between Sir Andrew and the red knight. The red knight's lance breaks on the shield, and he is thrown from his horse. His helmet falls off and everyone realizes that the knight is Roderick the guard, rather than Hugh. Fighting breaks out in the crowd between the supporters of the Duke of York and their enemies.
Beth and Patrick quickly escape to the cave, along with James and Sir Andrew. As Sir Andrew is about to give the cousins Mr. Whittaker's ring so that they can travel home, Hugh appears and steals the ring. The Imagination Station appears, and Hugh leaps in before anyone can stop him. Beth and Patrick reopen the Imagination Station door, but Hugh is gone. The cousins are able to travel back to Whit's End, however. They explain everything to Mr. Whittaker, and he realizes that leaving his ring in medieval England confused the Imagination Station. It thought he was still in the past, which is why it wouldn't allow him to travel.