The Hunters discover a secret room in the house; a clue sends Tess on the trail of a mysterious figure.
When Max gets into trouble at school, the headmistress decides she must speak with his foster parents; the children scramble to distract the headmistress from their foster parents' absence.
Max and Tess track the mysterious Rinus through Amsterdam and discover a cryptic notebook. Meanwhile, back at the Hunter house, the other kids make a mess as they try to cheer themselves up.
Max and Tess follow a clue to an Amsterdam cemetery where they uncover an old family secret and an unknown key; Anika has suspicions about the new cleaning lady.
The Hunter children put on a diamond exhibition to lure their nemesis out of hiding.
A mysterious message helps the Hunter children track down the elusive "Magpie," but it may be too late as others suspect the children are home alone.
When distant relatives, Aunt Hedwig and Uncle Eugene, are brought in to look after the Hunter children, the children worry their relatives are more interested in the Hunter fortune than helping the family.
The kids attempt a high-stakes exchange to rescue their parents while trying to outwit Aunt Hedwig and Uncle Eugene.
A new foster couple temporarily take in Anika and Sal. Meanwhile, Max and Tess uncover news about the blue diamond.
The kids begin to unravel the mystery around the missing red diamond. Meanwhile, Max keeps a secret from his siblings.
When the discovery of an old map points the kids toward the red diamond, Max makes a secret deal with a nemesis.
Tess and Max work together to trick their foes. Meanwhile, Anika doesn't want to celebrate her birthday.
Tess journeys into a maze of underground tunnels to find the red diamond. Meanwhile, Max has to conquer his fears; the kids get a surprise visit; and Max and Tess meet with the hooded figure.
Max tracks a possible suspect while the other children search the house for new clues.
The Hunter children capture a surprising trespasser and must hide their new guest.
The Hunter children must band together to outwit one of their biggest opponents.
The Hunters make a huge breakthrough but also suffer a big setback.
The kids find the stolen puzzle box and try to hide it from the police; Erik returns home, but he acts suspiciously; Daniel finds a clue in the museum footage; Max leaves for Spain.
The Hunters meet a young girl named Evie whose necklace has the same crown symbol as the cryptex; Erik reveals that he has a special connection to Evie.
The children find evidence that could prove Erik's innocence, but someone hacks into Sal's computer and erases the footage; Max realizes there is something strange about how his birth parents tracked him down.
Evie's stepmother, Lucia, tries to take her back; the Hunters attempt to track down information on Lucia by visiting Evie's old neighborhood, where they meet tech-loving Jennie and obtain footage that gives them their next clue.
When Tess finds an old Ruybosch family tree, she realizes that it bears the same crown symbol that they found on the necklace; Jenny helps Sal investigate the mysterious hacker; a comic book publishing company approaches Jake with a tempting offer.
The children think Lucia framed Erik and decide to frame her back by planting a vase in her house, but the plan goes awry and they must work to avoid getting caught.
The Hunter children attempt to make it out of Lucia's house with the vase before being caught.
The Hunter children devise a plan after realizing one of the relatives is a mole; Daniel can no longer work on the case when the police collect enough evidence to prosecute Erik.
The Hunter children confront Daniel, who reveals a new piece of evidence; Anika unwittingly uncovers the next piece of the puzzle.
The Hunters approach Detective Simon with their evidence, but are dismissed; Apollo appears with news that he can find the hacker; a daring mission leads the children to an abandoned railway yard.
Inside the hacker hideout, the children discover a map to a hidden room in the Hunter house; Max uses a telegraph machine to send a warning.
Apollo reveals the hacker's identity and plan; Anika and Jake attempt to contact Max; the Hunters confront the thief, but he escapes and locks them in the secret room.
Sal and Jennie discover the hacker's identity; Evie finds a mysterious object while searching for the crown.
Lucia makes another claim for Evie; Tess recognizes the object Evie found and thinks it will lead them to the true identity of the mole.
Tess and Sal work on discovering the mole's identity; Daniel and Anika spy on Lucia; the hacker learns he has been tricked.
Diane shows up and takes the crown when Erik is forced to go to the police station; the children finally uncover the mole's true identity.